fix(providers): show warning for all 400 errors before fallback

Map HTTP 400 to FailoverModelInvalid in classifyByStatus so ALL 400
errors trigger a warning message to the user and then fallback to the
next model. This prevents 400 errors with unrecognized messages from
being silently swallowed during fallback.

pkg/providers/error_classifier.go

@@ -182,11 +182,13 @@ func ClassifyError(err error, provider, model string) *FailoverError {
 }
 
 // classifyByStatus maps HTTP status codes to FailoverReason.
-// NOTE: 400 is intentionally NOT mapped here. Status 400 is too broad
-// (model-invalid, format errors, unknown API errors all return 400).
-// Instead, 400 errors are classified by message patterns in classifyByMessage.
+// NOTE: 400 maps to FailoverModelInvalid (retriable) — not FailoverFormat.
+// This ensures all 400 errors show a warning and fallback to the next model.
+// Specific 400 patterns (like model-invalid) are caught earlier by message matching.
 func classifyByStatus(status int) FailoverReason {
 	switch {
+	case status == 400:
+		return FailoverModelInvalid
 	case status == 401 || status == 403:
 		return FailoverAuth
 	case status == 402:

pkg/providers/error_classifier_test.go

@@ -41,7 +41,7 @@ func TestClassifyError_StatusCodes(t *testing.T) {
 		{402, FailoverBilling},
 		{408, FailoverTimeout},
 		{429, FailoverRateLimit},
-		// 400 is intentionally NOT here - classified by message patterns, not status code
+		{400, FailoverModelInvalid},
 		{500, FailoverTimeout},
 		{502, FailoverTimeout},
 		{503, FailoverTimeout},