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.test.ts

@@ -149,6 +149,28 @@ describe("model-error-classifier", () => {
     //#then
     expect(result).toBe(false)
   })
+
+  test("treats 'bad request' message as retryable (GitHub Copilot rolling update)", () => {
+    //#given
+    const error = { message: "400 Bad Request" }
+
+    //#when
+    const result = shouldRetryError(error)
+
+    //#then
+    expect(result).toBe(true)
+  })
+
+  test("treats 'bad request' lowercase as retryable", () => {
+    //#given
+    const error = { message: "bad request: model temporarily unavailable" }
+
+    //#when
+    const result = shouldRetryError(error)
+
+    //#then
+    expect(result).toBe(true)
+  })
 })
 
 export {}

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",