fix: delegate to MessageStream in _MessageStreamManager.__enter__

[youest]

fixes #2467

Problem

The _MessageStreamManager.__enter__() wrapper was calling __api_request() which returns a low-level anthropic.Stream object instead of the high-level anthropic.lib.streaming.MessageStream object.

This caused AttributeError: 'Stream' object has no attribute 'get_final_message' when users tried to call helper methods like get_final_message(), get_final_text(), or until_done() on instrumented streams.

Solution

Changed __enter__() to delegate to the wrapped MessageStreamManager.__enter__() method, which returns the proper MessageStream object with all helper methods intact.

Before:

def __enter__(self) -> Iterator[str]:
    raw = self.__api_request()  # Returns low-level Stream
    return _MessagesStream(raw, self._self_with_span)

After:

def __enter__(self) -> "MessageStream":
    # Delegate to the wrapped MessageStreamManager's __enter__
    message_stream = self.__wrapped__.__enter__()
    return _MessagesStream(message_stream, self._self_with_span)

Impact

This fix allows users to:

• Use stream.get_final_message() after consuming the stream
• Access other helper methods like get_final_text() and until_done()
• Use CrewAI framework's Anthropic integration without errors
• Follow standard Anthropic streaming patterns with instrumentation enabled

Testing

The existing streaming tests in test_instrumentor.py verify that basic streaming functionality works. This fix maintains backward compatibility while adding support for the missing helper methods.

---

Note

Return the real MessageStream from MessageStreamManager.__enter__ and pass through __exit__, enabling helper methods and proper cleanup; add tests and cassette to verify.

• Instrumentation (Anthropic):
  • Stream manager enter/exit:
    • Change _MessageStreamManager.__enter__() to call wrapped MessageStreamManager.__enter__() and return _MessagesStream wrapping the real MessageStream.
    • Add _MessagesStream.__exit__() to delegate to the underlying manager's __exit__ for proper cleanup.
  • TYPE_CHECKING: include MessageStream type.
• Tests:
  • Update test_anthropic_instrumentation_stream_message to assert stream.get_final_message() works.
  • Add test_anthropic_instrumentation_stream_context_manager_exit to verify manager __exit__ is called and get_final_message() works.
  • Add VCR cassette tests/.../test_anthropic_instrumentation_stream_context_manager_exit.yaml.

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