test(02-02): add failing tests for PlatformAdapter base class

- Add test for abstract class instantiation prevention
- Add test for incomplete implementation detection
- Add test for complete implementation instantiation
- Add test for async method signatures
- Add shared fixtures for platform tests

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

kb_pipeline/platforms/__init__.py

@@ -0,0 +1,21 @@
+"""Platform Adapters for KB Pipeline
+
+This module provides platform-specific adapters for content extraction
+from various platforms (Bilibili, Zhihu, etc.).
+
+Each adapter implements the PlatformAdapter interface:
+- get_content_type: Identify content type from URL
+- fetch_metadata: Fetch content metadata
+- fetch_content: Fetch content body in Markdown format
+- get_output_filename: Generate output filename
+"""
+
+from .base import PlatformAdapter
+
+# BilibiliAdapter will be imported when implemented
+# from .bilibili import BilibiliAdapter
+
+__all__ = [
+    "PlatformAdapter",
+    # "BilibiliAdapter",  # Uncomment when implemented
+]