fix: add Alibaba/DashScope rate-limit pattern to error classifier

Port from anomalyco/opencode#21355: Alibaba's DashScope API returns a
unique throttling message ('Request rate increased too quickly...') that
doesn't match standard rate-limit patterns ('rate limit', 'too many
requests'). This caused Alibaba errors to fall through to the 'unknown'
category rather than being properly classified as rate_limit with
appropriate backoff/rotation.

Add 'rate increased too quickly' to _RATE_LIMIT_PATTERNS and test with
the exact error message observed from the Alibaba provider.

agent/error_classifier.py

@@ -122,6 +122,7 @@ _RATE_LIMIT_PATTERNS = [
     "try again in",
     "please retry after",
     "resource_exhausted",
+    "rate increased too quickly",  # Alibaba/DashScope throttling
 ]
 
 # Usage-limit patterns that need disambiguation (could be billing OR rate_limit)

tests/agent/test_error_classifier.py

@@ -271,6 +271,22 @@ class TestClassifyApiError:
         assert result.reason == FailoverReason.rate_limit
         assert result.should_fallback is True
 
+    def test_alibaba_rate_increased_too_quickly(self):
+        """Alibaba/DashScope returns a unique throttling message.
+
+        Port from anomalyco/opencode#21355.
+        """
+        msg = (
+            "Upstream error from Alibaba: Request rate increased too quickly. "
+            "To ensure system stability, please adjust your client logic to "
+            "scale requests more smoothly over time."
+        )
+        e = MockAPIError(msg, status_code=400)
+        result = classify_api_error(e)
+        assert result.reason == FailoverReason.rate_limit
+        assert result.retryable is True
+        assert result.should_rotate_credential is True
+
     # ── Server errors ──
 
     def test_500_server_error(self):