fix(providers): make generic 400 errors retriable and fix double error message

Remove blanket 400 → FailoverFormat mapping from classifyByStatus.
Status 400 is too broad - model-invalid, format, and unknown errors all
return 400. Now 400 errors are classified by message patterns only:
- modelInvalidPatterns → FailoverModelInvalid (retriable + warning)
- formatPatterns → FailoverFormat (non-retriable)
- no match → unknown (retriable, allows fallback)

Also fix double error message: Run() was sending a duplicate error to
the user channel when runAgentLoop had already sent the ⚠️ error.

pkg/agent/loop.go

@@ -169,7 +169,12 @@ func (al *AgentLoop) Run(ctx context.Context) error {
 
 			response, err := al.processMessage(ctx, msg)
 			if err != nil {
-				response = fmt.Sprintf("Error processing message: %v", err)
+				// Error already sent to user channel by runAgentLoop, just log here
+				logger.ErrorCF("agent", "Message processing failed", map[string]any{
+					"error":   err.Error(),
+					"channel": msg.Channel,
+					"chat_id": msg.ChatID,
+				})
 			}
 
 			if response != "" {

pkg/providers/error_classifier.go

@@ -182,6 +182,9 @@ 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.
 func classifyByStatus(status int) FailoverReason {
 	switch {
 	case status == 401 || status == 403:
@@ -192,8 +195,6 @@ func classifyByStatus(status int) FailoverReason {
 		return FailoverTimeout
 	case status == 429:
 		return FailoverRateLimit
-	case status == 400:
-		return FailoverFormat
 	case transientStatusCodes[status]:
 		return FailoverTimeout
 	}

pkg/providers/error_classifier_test.go

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