Split AIFunction into a base class

[stephentoub]

• Adds AIFunctionDefinition as a base class for AIFunction, where AIFunction layers on invocability over the metadata exposed by the base.
• Added AIFunctionFactory.CreateDefinition and AIFunction.AsDefinitionOnly for creating AIFunctionDefinitions that aren't invocable.
• Updated FunctionInvokingChatClient to allow non-AIFunction call requests to pass through.
• Added FunctionInvokingChatClient.TerminateOnUnknownCalls to control behavior when requests are made to unknown functions.

Closes #6688

Microsoft Reviewers: Open in CodeFlow

[SteveSandersonMS]

Looks great! I think this is a perfectly clear distinction between definition/implementation.

[shyamnamboodiripad]

Thanks much for adding! The only other thing that may be nice to include is a simple helper / API to construct definitions for non-invokable tools (such as server-side tools).

[Copilot]

Pull Request Overview

This PR splits AIFunction into a base class hierarchy by introducing AIFunctionDefinition as a new base class. The main purpose is to enable scenarios where function metadata is needed without invocability, allowing better separation of concerns between function definition and execution.

• Introduces AIFunctionDefinition as a base class containing only metadata properties (Name, Description, JsonSchema, ReturnJsonSchema)
• Updates AIFunction to inherit from AIFunctionDefinition while maintaining invocation capabilities
• Adds support for non-invocable function definitions in FunctionInvokingChatClient with new TerminateOnUnknownCalls property

Reviewed Changes

Copilot reviewed 34 out of 34 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
src/Libraries/Microsoft.Extensions.AI.Abstractions/Functions/AIFunctionDefinition.cs	New base class containing function metadata properties
src/Libraries/Microsoft.Extensions.AI.Abstractions/Functions/AIFunction.cs	Updated to inherit from AIFunctionDefinition and added AsDefinitionOnly() method
src/Libraries/Microsoft.Extensions.AI.Abstractions/Functions/AIFunctionFactory.cs	Added CreateDefinition method for creating non-invocable function definitions
src/Libraries/Microsoft.Extensions.AI/ChatCompletion/FunctionInvokingChatClient.cs	Enhanced to handle non-invocable tools with new TerminateOnUnknownCalls property
Multiple OpenAI integration files	Updated method signatures to accept AIFunctionDefinition instead of AIFunction
Test files	Added comprehensive test coverage for new functionality

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[shyamnamboodiripad]

[stephentoub]

@SteveSandersonMS, would you mind taking another look? In particular at the FICC logic changes. Thanks.

[eiriktsarpalis]

Other than misgivings about the choice of name, this is a good change.