refactor: Switch to GitHub Search Issues API for PR queries

[ghinks]

Summary

Replace the /repos/{owner}/{repo}/pulls endpoint with the /search/issues API to enable server-side date range filtering for more efficient PR queries.

Changes

API Endpoint Update:

• Changed from /repos/{owner}/{repo}/pulls to /search/issues
• Added search query with date range: repo:owner/repo is:pr created:YYYY-MM-DD..YYYY-MM-DD
• Updated request parameters to use q, sort, and order instead of state, sort, direction

Response Handling:

• Parse Search API format {"items": [...]} instead of direct array response
• Added type guard (isinstance) for safe dictionary access
• Updated _make_pr_request_with_retry() return type to dict[str, Any] | list[dict]

Code Simplification:

• Removed client-side date filtering loop (lines 142-147) - API now handles filtering
• Set reached_boundary = True since Search API always returns complete results
• Eliminated unnecessary datetime parsing and comparison logic

Test Updates:

• Updated test mocks to return Search API response format
• Added side_effect to simulate pagination properly
• Adjusted assertions to expect server-side filtered results (2 PRs instead of 3)
• Added mock headers to prevent rate limiting test issues

Benefits

✅ Server-side date filtering - GitHub API handles date range filtering, no client-side processing needed
✅ Reduced data transfer - Only relevant PRs returned, not all PRs
✅ Faster queries - More efficient for repositories with many PRs
✅ Simpler code - Removed ~20 lines of client-side filtering logic
✅ All tests passing - 60/60 tests pass
✅ Type safe - Full mypy compliance

Trade-offs

⚠️ Result limit - Search API capped at 1000 results (10 pages × 100/page) vs unlimited pagination
⚠️ Lower rate limit - 30 requests/min vs 60 requests/min for standard API
⚠️ Indexing delay - Potential slight delays (typically < 1 minute) for very recent PRs

Test Plan

• All 60 unit tests passing
• Ruff linting passed
• Mypy type checking passed (24 source files)
• Verified mock responses match Search API format
• Confirmed pagination error handling works correctly

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