[WIP] Fix OnIntermediateMessage callback behavior - Always yield responses

[Copilot]

Summary

Fix the callback behavior issues where responses are not yielded when options.OnIntermediateMessage is null. The method should yield responses regardless of the callback status to maintain consistency with previous behavior.

Issues to Address

• Issue When options.OnIntermediateMessage is null, the method does not yield any response, which differs from the previous behavior that always yielded a response. It is recommended to yield responses regardless of the callback, for example, by yielding the responseItem in an else branch. #752: Non-streaming method not yielding responses when callback is null
• Issue When options.OnIntermediateMessage is null, the method does not yield any response, which differs from the previous behavior that always yielded a response. It is recommended to yield responses regardless of the callback, for example, by yielding the responseItem in an else branch. #751: Duplicate of When options.OnIntermediateMessage is null, the method does not yield any response, which differs from the previous behavior that always yielded a response. It is recommended to yield responses regardless of the callback, for example, by yielding the responseItem in an else branch. #752
• Issue Similar to the non-streaming implementation, the InvokeStreamingAsync method only yields a streaming response when the OnIntermediateMessage callback is provided. Consider yielding the streaming response even if the callback is null to maintain consistency with previous behavior. #750: Streaming method not yielding responses when callback is null

Implementation Requirements

1. For non-streaming implementation: Ensure the method yields a response even when options.OnIntermediateMessage is null
2. For streaming implementation: Ensure InvokeStreamingAsync yields streaming responses even when the callback is null
3. Maintain backward compatibility: Ensure existing functionality continues to work

Expected Changes

• Add an else branch to yield responseItem when callback is null
• Apply similar logic to both streaming and non-streaming implementations
• Add unit tests to verify the new behavior
• Update documentation if necessary

Acceptance Criteria

• Both streaming and non-streaming methods yield responses when callback is null
• Existing callback functionality remains unchanged
• Unit tests pass
• No breaking changes to API

Files Likely to be Modified

• Streaming response handling code
• Non-streaming response handling code
• Related unit tests

[Copilot]

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