#9427: fix: trigger model fallback on all 4xx HTTP errors

## Summary Fixes #9297 When an LLM provider responds with a 4xx HTTP error, the system now falls back to the configured alternative model instead of returning the raw error to the user. ## Problem Previously, only specific 4xx status codes (401, 402, 403, 408, 429) would trigger model fallback. Other 4xx errors like: - 400 Bad Request (e.g., model not found, invalid endpoint) - 404 Not Found (e.g., non-existent endpoint) ...would bubble up directly to the user without attempting the fallback. ## Solution Extended `resolveFailoverReasonFromError()` to treat all 4xx client errors as fallback-worthy. This allows the system to try alternative models when the primary provider returns any client error. ## Changes - `src/agents/failover-error.ts`: Added catch-all for 4xx status codes - `src/agents/model-fallback.test.ts`: Added tests for 400 and 404 fallback scenarios ## Testing - Added unit tests for 400 Bad Request fallback - Added unit tests for 404 Not Found fallback <!-- greptile_comment --> <h2>Greptile Overview</h2> <h3>Greptile Summary</h3> This PR broadens model failover triggering by treating any HTTP 4xx response from an LLM provider as failover-worthy, and adds unit coverage for 400/404 scenarios in `runWithModelFallback`. Core behavior change is implemented in `src/agents/failover-error.ts` via `resolveFailoverReasonFromError()`, which now maps previously-unhandled 4xx status codes into a failover reason that `runWithModelFallback` recognizes, causing it to try configured fallback models instead of surfacing the raw provider error. <h3>Confidence Score: 3/5</h3> - This PR is moderately safe to merge once 4xx-to-reason mapping is corrected to avoid misclassifying format errors as auth. - Change is small and covered by tests for 400/404 fallback, but the implementation reuses the "auth" reason for all 4xx, which can incorrectly affect downstream cooldown/telemetry and user-visible error handling for real 400-format failures. - src/agents/failover-error.ts <!-- greptile_other_comments_section --> <!-- /greptile_comment -->