Fix: Properly populate AssistantMessage error field (#506)

## Summary
Fixes #505
- Fix `AssistantMessage.error` field not being populated due to
incorrect data path
- The `error` field is at the top level of the response
(`data["error"]`), not nested inside `data["message"]["error"]`
- Add test coverage for error scenarios (authentication_failed, unknown,
rate_limit)
## Test plan
- [x] Added unit tests for assistant messages without errors
- [x] Run `python -m pytest tests/test_message_parser.py` to verify all
tests pass
  🤖 Generated with [Claude Code](https://claude.com/claude-code)

Author: ben-complement

src/claude_agent_sdk/_internal/message_parser.py

@@ -126,7 +126,7 @@ def parse_message(data: dict[str, Any]) -> Message:
                     content=content_blocks,
                     model=data["message"]["model"],
                     parent_tool_use_id=data.get("parent_tool_use_id"),
-                    error=data["message"].get("error"),
+                    error=data.get("error"),
                 )
             except KeyError as e:
                 raise MessageParseError(

tests/test_message_parser.py

@@ -363,3 +363,77 @@ def test_message_parse_error_contains_data(self):
         with pytest.raises(MessageParseError) as exc_info:
             parse_message(data)
         assert exc_info.value.data == data
+
+    def test_parse_assistant_message_without_error(self):
+        """Test that assistant message without error has error=None."""
+        data = {
+            "type": "assistant",
+            "message": {
+                "content": [{"type": "text", "text": "Hello"}],
+                "model": "claude-opus-4-5-20251101",
+            },
+        }
+        message = parse_message(data)
+        assert isinstance(message, AssistantMessage)
+        assert message.error is None
+
+    def test_parse_assistant_message_with_authentication_error(self):
+        """Test parsing assistant message with authentication_failed error.
+
+        The error field is at the top level of the data, not inside message.
+        This matches the actual CLI output format.
+        """
+        data = {
+            "type": "assistant",
+            "message": {
+                "content": [
+                    {"type": "text", "text": "Invalid API key · Fix external API key"}
+                ],
+                "model": "<synthetic>",
+            },
+            "session_id": "test-session",
+            "error": "authentication_failed",
+        }
+        message = parse_message(data)
+        assert isinstance(message, AssistantMessage)
+        assert message.error == "authentication_failed"
+        assert len(message.content) == 1
+        assert isinstance(message.content[0], TextBlock)
+
+    def test_parse_assistant_message_with_unknown_error(self):
+        """Test parsing assistant message with unknown error (e.g., 404, 500).
+
+        When the CLI encounters API errors like model not found or server errors,
+        it sets error to 'unknown' and includes the error details in the text content.
+        """
+        data = {
+            "type": "assistant",
+            "message": {
+                "content": [
+                    {
+                        "type": "text",
+                        "text": 'API Error: 500 {"type":"error","error":{"type":"api_error","message":"Internal server error"}}',
+                    }
+                ],
+                "model": "<synthetic>",
+            },
+            "session_id": "test-session",
+            "error": "unknown",
+        }
+        message = parse_message(data)
+        assert isinstance(message, AssistantMessage)
+        assert message.error == "unknown"
+
+    def test_parse_assistant_message_with_rate_limit_error(self):
+        """Test parsing assistant message with rate_limit error."""
+        data = {
+            "type": "assistant",
+            "message": {
+                "content": [{"type": "text", "text": "Rate limit exceeded"}],
+                "model": "<synthetic>",
+            },
+            "error": "rate_limit",
+        }
+        message = parse_message(data)
+        assert isinstance(message, AssistantMessage)
+        assert message.error == "rate_limit"