fix(error_classifier): classify 'overloaded' as FailoverReason.overloaded before rate_limit

When a provider (e.g. Z.AI) returns 'The service may be temporarily
overloaded, please try again later' as HTTP 200 or HTTP 400, the error
was matched against _RATE_LIMIT_PATTERNS (which includes
'servicequotaexceededexception') and classified as rate_limit with
should_rotate_credential=True. After 2 failures the single API key was
marked exhausted and all further retries failed.

The fix adds an 'overloaded' / 'temporarily overloaded' pattern check
BEFORE the rate_limit check in both _classify_400 and
_classify_by_message. Overloaded errors now get FailoverReason.overloaded
(retryable, should_fallback) instead of rate_limit, preventing
unnecessary credential rotation.

Closes #14038

agent/error_classifier.py

@@ -590,6 +590,16 @@ def _classify_400(
 
     # Some providers return rate limit / billing errors as 400 instead of 429/402.
     # Check these patterns before falling through to format_error.
+
+    # Overloaded patterns — server-side overload, NOT a credential/billing issue.
+    # Must come before rate_limit check to avoid rotating credentials unnecessarily.
+    if "overloaded" in error_msg or "temporarily overloaded" in error_msg:
+        return result_fn(
+            FailoverReason.overloaded,
+            retryable=True,
+            should_fallback=True,
+        )
+
     if any(p in error_msg for p in _RATE_LIMIT_PATTERNS):
         return result_fn(
             FailoverReason.rate_limit,
@@ -723,7 +733,14 @@ def _classify_by_message(
             should_fallback=True,
         )
 
-    # Rate limit patterns
+    # Rate limit patterns — but overloaded must come first to avoid credential rotation.
+    if "overloaded" in error_msg or "temporarily overloaded" in error_msg:
+        return result_fn(
+            FailoverReason.overloaded,
+            retryable=True,
+            should_fallback=True,
+        )
+
     if any(p in error_msg for p in _RATE_LIMIT_PATTERNS):
         return result_fn(
             FailoverReason.rate_limit,