.NET: Pass AdditionalProperties from parent to child when exposing an agent as a FunctionTool

[westey-m]

Motivation and Context

#3127

Description

• Pass AdditionalProperties from parent to child when exposing an agent as a FunctionTool

Contribution Checklist

• The code builds clean without any errors or warnings
• The PR follows the Contribution Guidelines
• All unit tests pass, and I have added new tests where possible
• Is this a breaking change? If yes, add "[BREAKING]" prefix to the title of the PR.

[Copilot]

Pull request overview

This PR enables the propagation of AdditionalProperties from a parent agent to a child agent when the child is exposed as a FunctionTool. This allows custom properties set on the parent agent's run context to flow through to nested agent invocations.

Changes:

• Modified AgentExtensions.AsAIFunction to extract AdditionalProperties from the parent agent's FunctionInvokingChatClient context and pass them to the child agent via AgentRunOptions
• Added a comprehensive unit test to verify that AdditionalProperties are correctly passed from parent to child agent
• Added a helper method in the test class to use reflection for setting the FunctionInvokingChatClient's CurrentContext for testing purposes

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 2 comments.

File	Description
dotnet/src/Microsoft.Agents.AI/AgentExtensions.cs	Modified InvokeAgentAsync to propagate AdditionalProperties from FunctionInvokingChatClient.CurrentContext to child agent options
dotnet/tests/Microsoft.Agents.AI.UnitTests/AgentExtensionsTests.cs	Added test for AdditionalProperties propagation and helper method for setting CurrentContext via reflection; enhanced TestAgent to track received AgentRunOptions