fix: address CodeRabbit review - extract rate limit logic to shared module

- Create rate_limit_utils.py with shared constants and functions
- Update agent.py to import from shared module
- Update test_agent.py to import from shared module (removes duplication)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

rate_limit_utils.py

@@ -0,0 +1,69 @@
+"""
+Rate Limit Utilities
+====================
+
+Shared utilities for detecting and handling API rate limits.
+Used by both agent.py (production) and test_agent.py (tests).
+"""
+
+import re
+from typing import Optional
+
+# Rate limit detection patterns (used in both exception messages and response text)
+RATE_LIMIT_PATTERNS = [
+    "limit reached",
+    "rate limit",
+    "rate_limit",
+    "too many requests",
+    "quota exceeded",
+    "please wait",
+    "try again later",
+    "429",
+    "overloaded",
+]
+
+
+def parse_retry_after(error_message: str) -> Optional[int]:
+    """
+    Extract retry-after seconds from various error message formats.
+
+    Handles common formats:
+    - "Retry-After: 60"
+    - "retry after 60 seconds"
+    - "try again in 5 seconds"
+    - "30 seconds remaining"
+
+    Args:
+        error_message: The error message to parse
+
+    Returns:
+        Seconds to wait, or None if not parseable.
+    """
+    patterns = [
+        r"retry.?after[:\s]+(\d+)\s*(?:seconds?)?",
+        r"try again in\s+(\d+)\s*(?:seconds?|s\b)",
+        r"(\d+)\s*seconds?\s*(?:remaining|left|until)",
+    ]
+
+    for pattern in patterns:
+        match = re.search(pattern, error_message, re.IGNORECASE)
+        if match:
+            return int(match.group(1))
+
+    return None
+
+
+def is_rate_limit_error(error_message: str) -> bool:
+    """
+    Detect if an error message indicates a rate limit.
+
+    Checks against common rate limit patterns from various API providers.
+
+    Args:
+        error_message: The error message to check
+
+    Returns:
+        True if the message indicates a rate limit, False otherwise.
+    """
+    error_lower = error_message.lower()
+    return any(pattern in error_lower for pattern in RATE_LIMIT_PATTERNS)