fix: handle HTTP 529 (Anthropic overloaded) in failover error classification

Classify Anthropic's 529 status code as "rate_limit" so model fallback
triggers reliably without depending on fragile message-based detection.

Closes #28502

src/agents/failover-error.ts

@@ -178,6 +178,9 @@ export function resolveFailoverReasonFromError(err: unknown): FailoverReason | n
   if (status === 502 || status === 503 || status === 504) {
     return "timeout";
   }
+  if (status === 529) {
+    return "rate_limit";
+  }
   if (status === 400) {
     return "format";
   }