Allow FunctionResultContent pass-through when CallId matches

[Copilot]

• Understand the problem: FunctionInvokingChatClient.CreateResponseMessages should reuse FunctionResultContent returned by the function if the CallId matches
• Implement the change in FunctionInvokingChatClient.cs at line 1230
• Add tests to verify the new behavior
• Unseal FunctionCallContent and FunctionResultContent classes
• Update API baseline to reflect unsealed classes
• Add test validating that a derived FunctionResultContent type is propagated to the inner IChatClient
• Validate that the FunctionResultContent is the only one in the tool message
• Assert reference equality in tests
• Add streaming path tests for all new tests
• Obtain tool message from ChatResponse instead of capturing via callback
• Run tests to validate changes
• Code review fixes

Summary

This PR modifies FunctionInvokingChatClient.CreateResponseMessages to check if the function result is already a FunctionResultContent with a matching CallId. If so, it uses that instance directly instead of wrapping it in a new FunctionResultContent. This gives the AIFunction flexibility to create its own FunctionResultContent configured however it wants.

Additionally, this PR unseals FunctionCallContent and FunctionResultContent classes to allow derivation.

Changes:

1. Modified FunctionInvokingChatClient.CreateFunctionResultContent method to check if the result is a FunctionResultContent with a matching CallId and return it directly
2. Unsealed FunctionCallContent and FunctionResultContent classes to allow derivation
3. Updated API baseline files for the unsealed classes
4. Added tests with both non-streaming and streaming paths and reference equality assertions:
   • FunctionReturningFunctionResultContentWithMatchingCallId_UsesItDirectly: Verifies that when a function returns a FunctionResultContent with matching CallId, it is used directly (same reference)
   • FunctionReturningFunctionResultContentWithMismatchedCallId_WrapsIt: Verifies that when a function returns a FunctionResultContent with mismatched CallId, it is wrapped in a new FunctionResultContent (with the inner one reference-equal)
   • FunctionReturningDerivedFunctionResultContent_PropagatesInstanceToInnerClient: Verifies that a derived FunctionResultContent type is properly propagated to the wrapped IChatClient and is the only one in the tool message (same reference)

Original prompt

On FunctionInvokingChatClient.cs:1230-1230, Right now, FunctionInvokingChatClient.CreateResponseMessages wraps the result of the function in a new FunctionResultContent. If, however, the object returned by the function is already a FunctionResultContent, and if that FunctionResultContent's CallId matches the one it's supposed to be, the implementation should just use that returned instance rather than creating a new one. This gives the AIFunction flexibility to create its own FunctionResultContent configured however it wants.

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Microsoft Reviewers: Open in CodeFlow

[stephentoub]

@copilot also unseal FunctionCallContent and FunctionResultContent and add tests validating that an AIFunction can return a FunctionResultContent-derived type and that instance is what's propagated to the IChatClient wrapped by the FunctionInvokingChatClient.

[Copilot]

@copilot also unseal FunctionCallContent and FunctionResultContent and add tests validating that an AIFunction can return a FunctionResultContent-derived type and that instance is what's propagated to...

Done in 790f979. Unsealed both FunctionCallContent and FunctionResultContent, updated the API baseline file, and added the FunctionReturningDerivedFunctionResultContent_PropagatesInstanceToInnerClient test that verifies a derived FunctionResultContent type is properly propagated to the wrapped IChatClient.

[jozkee]

Otherwise, LGTM.