feat(ai-client): 实现多 AI 提供商支持与 fallback 机制

- 支持 MiniMax、智谱、通义千问等多家 AI 提供商
- 实现自动 fallback 机制（主失败则自动切换备用）
- 修复 MiniMax Anthropic 格式响应解析
- 修复智谱 API 端点（自动添加 /chat/completions）
- 新增 _call_api_with_fallback 方法统一处理 fallback 逻辑
- 更新 _load_ai_config 加载主配置和 fallbacks 配置

配置文件位置: ~/.config/obsidian-kb/config.yaml

skills/obsidian-kb/src/ai_client.py

@@ -1,9 +1,10 @@
 """Obsidian KB - AI客户端
-C5.3: 实现长视频分段分析逻辑
+支持多个 AI 提供商和 fallback 机制
 """
 import os
 import json
-from typing import Dict, Any, List, Optional
+import requests
+from typing import Dict, Any, List, Optional, Tuple
 from src.models import ContentType
 from src.config import get_config
 from src.content_chunker import ContentChunker, get_chunking_strategy, chunk_content
@@ -11,21 +12,217 @@ from src.content_chunker import ContentChunker, get_chunking_strategy, chunk_con
 
 class AIClient:
     """
-    AI客户端 - 支持分段分析长视频内容
+    AI客户端 - 支持分段分析长视频内容和 fallback 机制
     """
 
     def __init__(self):
         self.config = get_config()
         self.chunker = ContentChunker()
+        # 获取 AI 配置
+        self._load_ai_config()
+
+    def _load_ai_config(self) -> None:
+        """加载 AI 配置（主 provider + fallbacks）"""
+        self.ai_configs = []
+
+        # 加载主配置
+        main_provider = self.config.get('ai.provider', 'openai')
+        main_config = {
+            'provider': main_provider,
+            'api_key': self.config.get('ai.api_key'),
+            'model': self.config.get('ai.model', 'gpt-4'),
+            'base_url': self.config.get('ai.base_url'),
+            'is_primary': True
+        }
+        self.ai_configs.append(main_config)
+
+        # 加载 fallback 配置
+        fallbacks = self.config.get('ai.fallbacks', [])
+        for fb in fallbacks:
+            self.ai_configs.append({
+                'provider': fb.get('provider'),
+                'api_key': fb.get('api_key'),
+                'model': fb.get('model'),
+                'base_url': fb.get('base_url'),
+                'is_primary': False
+            })
+
+    def _call_api_with_fallback(self, prompt: str, max_tokens: int = 1000, response_format: str = 'text') -> Tuple[bool, str]:
+        """
+        使用 fallback 机制调用 API
+
+        Args:
+            prompt: 提示词
+            max_tokens: 最大 token 数
+            response_format: 响应格式 ('text' 或 'json')
+
+        Returns:
+            (success, response) 元组
+        """
+        last_error = None
+
+        for i, ai_config in enumerate(self.ai_configs):
+            try:
+                if not ai_config.get('api_key'):
+                    print(f"[AIClient] 跳过 {ai_config.get('provider')} - API Key 为空")
+                    continue
+
+                provider_type = ai_config.get('provider')
+                print(f"[AIClient] 尝试使用 {provider_type} (优先级 {i+1}/{len(self.ai_configs)})...")
+
+                response = self._call_single_provider(ai_config, prompt, max_tokens, response_format)
+                return True, response
+
+            except Exception as e:
+                last_error = e
+                print(f"[AIClient] {provider_type} 调用失败: {e}")
+                continue
+
+        # 所有 provider 都失败了
+        error_msg = f"所有 AI 提供商均失败，最后错误: {last_error}" if last_error else "没有可用的 AI 配置"
+        print(f"[AIClient] {error_msg}")
+        return False, self._mock_generate(prompt, max_tokens)  # 返回 mock 响应作为兜底
+
+    def _call_single_provider(self, ai_config: Dict, prompt: str, max_tokens: int, response_format: str) -> str:
+        """
+        调用单个 AI 提供商
+
+        Args:
+            ai_config: AI 配置
+            prompt: 提示词
+            max_tokens: 最大 token 数
+            response_format: 响应格式
+
+        Returns:
+            API 响应文本
+        """
+        provider = ai_config.get('provider')
+        api_key = ai_config.get('api_key')
+        model = ai_config.get('model')
+        base_url = ai_config.get('base_url')
+
+        if provider == 'minimax':
+            return self._call_minimax(api_key, model, base_url, prompt, max_tokens, response_format)
+        elif provider == 'zhipu':
+            return self._call_zhipu(api_key, model, base_url, prompt, max_tokens, response_format)
+        elif provider == 'alibaba':
+            return self._call_alibaba(api_key, model, base_url, prompt, max_tokens, response_format)
+        elif provider == 'openai':
+            return self._call_openai(api_key, model, base_url, prompt, max_tokens, response_format)
+        else:
+            raise ValueError(f"不支持的 provider: {provider}")
+
+    def _call_minimax(self, api_key: str, model: str, base_url: str, prompt: str, max_tokens: int, response_format: str) -> str:
+        """调用 MiniMax API (Anthropic Messages 兼容接口)"""
+        url = base_url or "https://api.minimaxi.com/anthropic/v1/messages"
+
+        headers = {
+            "Authorization": f"Bearer {api_key}",
+            "Content-Type": "application/json"
+        }
+
+        payload = {
+            "model": model,
+            "max_tokens": max_tokens,
+            "messages": [
+                {"role": "user", "content": prompt}
+            ]
+        }
+
+        if response_format == 'json':
+            payload['messages'][0]['content'] = prompt + "\n\n请直接返回 JSON 格式，不要有其他文字。"
+
+        response = requests.post(url, headers=headers, json=payload, timeout=30)
+        response.raise_for_status()
+        data = response.json()
+
+        # MiniMax 返回的是 Anthropic 格式，content 是一个数组
+        # 需要找到 type='text' 的元素
+        for item in data.get('content', []):
+            if item.get('type') == 'text':
+                return item.get('text', '')
+
+        # 如果没有找到 text 类型，返回第一个元素的值（兼容旧格式）
+        return data.get('content', [{}])[0].get('text', '')
+
+    def _call_zhipu(self, api_key: str, model: str, base_url: str, prompt: str, max_tokens: int, response_format: str) -> str:
+        """调用智谱 AI API (OpenAI 兼容接口)"""
+        # 确保 base_url 包含 /chat/completions
+        if base_url and not base_url.endswith('/chat/completions'):
+            url = f"{base_url}/chat/completions"
+        else:
+            url = base_url or "https://open.bigmodel.cn/api/paas/v4/chat/completions"
+
+        headers = {
+            "Authorization": f"Bearer {api_key}",
+            "Content-Type": "application/json"
+        }
+
+        payload = {
+            "model": model,
+            "max_tokens": max_tokens,
+            "messages": [
+                {"role": "user", "content": prompt}
+            ]
+        }
+
+        response = requests.post(url, headers=headers, json=payload, timeout=30)
+        response.raise_for_status()
+        data = response.json()
+        return data['choices'][0]['message']['content']
+
+    def _call_alibaba(self, api_key: str, model: str, base_url: str, prompt: str, max_tokens: int, response_format: str) -> str:
+        """调用阿里云通义千问 API (OpenAI 兼容接口)"""
+        url = f"{base_url}/chat/completions"
+
+        headers = {
+            "Authorization": f"Bearer {api_key}",
+            "Content-Type": "application/json"
+        }
+
+        payload = {
+            "model": model,
+            "max_tokens": max_tokens,
+            "messages": [
+                {"role": "user", "content": prompt}
+            ]
+        }
+
+        response = requests.post(url, headers=headers, json=payload, timeout=30)
+        response.raise_for_status()
+        data = response.json()
+        return data['choices'][0]['message']['content']
+
+    def _call_openai(self, api_key: str, model: str, base_url: str, prompt: str, max_tokens: int, response_format: str) -> str:
+        """调用 OpenAI API"""
+        url = base_url or "https://api.openai.com/v1/chat/completions"
+
+        headers = {
+            "Authorization": f"Bearer {api_key}",
+            "Content-Type": "application/json"
+        }
+
+        payload = {
+            "model": model,
+            "max_tokens": max_tokens,
+            "messages": [
+                {"role": "user", "content": prompt}
+            ]
+        }
+
+        response = requests.post(url, headers=headers, json=payload, timeout=30)
+        response.raise_for_status()
+        data = response.json()
+        return data['choices'][0]['message']['content']
 
     def quick_generate(self, prompt: str, max_tokens: int = 100) -> str:
         """快速生成（用于轻量扫描）"""
-        # 模拟AI响应（实际应调用API）
-        return self._mock_generate(prompt, max_tokens)
+        success, response = self._call_api_with_fallback(prompt, max_tokens, 'text')
+        return response
     
     def generate_json(self, prompt: str) -> Dict[str, Any]:
         """生成JSON格式响应"""
-        response = self.quick_generate(prompt, max_tokens=1000)
+        success, response = self._call_api_with_fallback(prompt, 1000, 'json')
         try:
             return json.loads(response)
         except: