fix(model): treat 'bad request' as retryable error for GitHub Copilot fallback (#3130)

GitHub Copilot performs rolling model updates which sometimes return
'400 Bad Request' when a model is temporarily unavailable. This error
was not in the retryable message patterns, causing model fallback to
not trigger and users getting stuck.

Changes:
- Added 'bad request' to RETRYABLE_MESSAGE_PATTERNS in model-error-classifier.ts
- Added test cases for 'bad request' pattern matching

Fixes #3130

src/shared/model-error-classifier.ts

@@ -52,6 +52,7 @@ const RETRYABLE_MESSAGE_PATTERNS = [
   "over limit",
   "overloaded",
   "bad gateway",
+  "bad request",
   "unknown provider",
   "provider not found",
   "model_not_supported",