#13820: feat(agents): retry empty-stream once before fallback

This PR improves resilience for transient empty-stream failures (`request ended without sending any chunks`) by retrying once on the same model before proceeding through fallback models. Changes: - classify empty-stream patterns as timeout failover reasons - add one-time in-model retry (300-800ms jitter, feature-flag controlled) - keep robust fallback behavior for remaining retryable errors - add unit tests for classification, retry success, retry->fallback, and feature-flag off path <!-- greptile_comment --> <h2>Greptile Overview</h2> <h3>Greptile Summary</h3> This PR enhanced resilience for transient empty-stream failures by implementing a one-time in-model retry before proceeding to fallback models. The implementation correctly classifies empty-stream errors (`request ended without sending any chunks`, `stream ended before first chunk`) as timeout failover reasons and adds retry logic with configurable jitter (300-800ms default, feature-flag controlled via `OPENCLAW_EMPTY_STREAM_RETRY`). Key changes: - Added empty-stream error patterns to timeout classification in error pattern matching - Implemented one-time in-model retry with exponential jitter before fallback - Added comprehensive test coverage for retry success, retry-then-fallback, and feature-flag disable scenarios - Maintained backward compatibility through feature flags and existing fallback behavior <h3>Confidence Score: 5/5</h3> - This PR is safe to merge with minimal risk - The implementation is well-designed with proper error classification, comprehensive test coverage covering all code paths (retry success, retry failure fallback, feature-flag disable), and backward-compatible feature flags. The retry logic is correctly isolated with a guard flag preventing infinite loops, and the delay mechanism uses appropriate jitter to prevent thundering herd issues. - No files require special attention <!-- greptile_other_comments_section --> _{(2/5) Greptile learns from your feedback when you react with thumbs up/down!} <!-- /greptile_comment -->