[Fix] Malformed messages returning 500 instead of 400

The existing AttributeError detection in proxy error handling only
checked one level deep in the exception chain (__cause__, __context__,
original_exception). In practice, the AttributeError from malformed
messages gets wrapped in multiple layers (AttributeError ->
OpenAIException -> APIConnectionError), so the check never found it.

Extracted the check into _has_attribute_error_in_chain() which walks
the full exception chain recursively (depth-capped at 10 to prevent
infinite loops from circular references).

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

tests/test_litellm/proxy/test_common_request_processing.py

@@ -15,6 +15,7 @@ from litellm.proxy.common_request_processing import (
     ProxyConfig,
     _extract_error_from_sse_chunk,
     _get_cost_breakdown_from_logging_obj,
+    _has_attribute_error_in_chain,
     _is_azure_model_router_request,
     _override_openai_response_model,
     _parse_event_data_for_error,
@@ -1701,3 +1702,50 @@ class TestDDSpanTaggerTagRequest:
             )
 
         mock_set_tag.assert_called_once_with("litellm.requested_model", "claude-3-5-sonnet")
+
+
+class TestHasAttributeErrorInChain:
+    """Tests for _has_attribute_error_in_chain helper."""
+
+    def test_direct_attribute_error(self):
+        exc = AttributeError("'str' object has no attribute 'get'")
+        assert _has_attribute_error_in_chain(exc) is True
+
+    def test_no_attribute_error(self):
+        exc = ValueError("some other error")
+        assert _has_attribute_error_in_chain(exc) is False
+
+    def test_attribute_error_in_cause(self):
+        inner = AttributeError("bad attribute")
+        outer = RuntimeError("wrapper")
+        outer.__cause__ = inner
+        assert _has_attribute_error_in_chain(outer) is True
+
+    def test_attribute_error_in_context(self):
+        inner = AttributeError("bad attribute")
+        outer = RuntimeError("wrapper")
+        outer.__context__ = inner
+        assert _has_attribute_error_in_chain(outer) is True
+
+    def test_attribute_error_in_original_exception(self):
+        inner = AttributeError("bad attribute")
+        outer = RuntimeError("wrapper")
+        outer.original_exception = inner  # type: ignore
+        assert _has_attribute_error_in_chain(outer) is True
+
+    def test_attribute_error_nested_two_levels(self):
+        """Simulates the real failure: AttributeError -> OpenAIException -> APIConnectionError."""
+        attr_err = AttributeError("'str' object has no attribute 'get'")
+        mid = Exception("OpenAIException wrapper")
+        mid.__context__ = attr_err
+        outer = Exception("APIConnectionError wrapper")
+        outer.__context__ = mid
+        assert _has_attribute_error_in_chain(outer) is True
+
+    def test_depth_limit_prevents_infinite_loop(self):
+        """Ensure circular references don't cause infinite recursion."""
+        exc_a = RuntimeError("a")
+        exc_b = RuntimeError("b")
+        exc_a.__context__ = exc_b
+        exc_b.__context__ = exc_a  # circular
+        assert _has_attribute_error_in_chain(exc_a) is False