feat(01-03): implement retry mechanism with tenacity

- retry_on_network_error decorator for automatic retry
- Max 3 attempts with exponential backoff (2s, 4s, max 10s)
- Only retries on NetworkError
- Logs retry attempts via before_sleep_log
- Reraises original exception after max attempts

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

kb_pipeline/retry.py

@@ -0,0 +1,51 @@
+"""Retry mechanism for KB Pipeline
+
+Provides automatic retry for network errors using tenacity library.
+"""
+
+import logging
+from tenacity import (
+    retry,
+    stop_after_attempt,
+    wait_exponential,
+    retry_if_exception_type,
+    before_sleep_log,
+)
+
+from .errors import NetworkError
+
+logger = logging.getLogger(__name__)
+
+retry_on_network_error = retry(
+    stop=stop_after_attempt(3),
+    wait=wait_exponential(multiplier=1, min=2, max=10),
+    retry=retry_if_exception_type(NetworkError),
+    before_sleep=before_sleep_log(logger, logging.WARNING),
+    reraise=True,
+)
+"""Decorator for automatic retry on network errors
+
+Automatically retries functions that raise NetworkError up to 3 times
+with exponential backoff (2s, 4s, up to max 10s).
+
+Example:
+    @retry_on_network_error
+    def fetch_data(url):
+        response = requests.get(url)
+        if response.status_code >= 500:
+            raise NetworkError(
+                step_name="fetch",
+                input_summary=f"url={url}",
+                status_code=response.status_code
+            )
+        return response.json()
+
+Args:
+    func: Function to wrap with retry logic
+
+Returns:
+    Wrapped function with retry logic
+
+Raises:
+    NetworkError: After 3 failed attempts
+"""