.NET: [BREAKING] Trim final FRC to match service storage

[westey-m]

Motivation and Context

#4791

Currently there is a mismatch in behavior between service stored chat history and ChatHistoryProvider stored chat history when function tools are terminated. This brings those behaviors into sync, with an opt out.
Also not notifying AIContextProviders to match chat history storage.

This change is a behavioral break, but only for very limited circumstances, e.g. when using function call termination.

Description

• Don't store FRC when it is the last content in the response, which means it was generated without a subsequent service call.

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

Aligns ChatHistoryProvider-backed chat history with service-side storage by trimming trailing FunctionResultContent when tool loops terminate, with an opt-out flag.

Changes:

• Added ChatClientAgentRunOptions.StoreFinalFunctionResultContent (experimental) to control persistence of trailing FunctionResultContent.
• Implemented response trimming of trailing tool-only FunctionResultContent before persisting to chat history (and currently also before AI context notification).
• Added unit tests covering non-streaming/streaming behavior and cloning behavior for the new run option.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 5 comments.

File	Description
dotnet/src/Microsoft.Agents.AI/ChatClient/ChatClientAgent.cs	Filters trailing tool FunctionResultContent before notifying history/context providers.
dotnet/src/Microsoft.Agents.AI/ChatClient/ChatClientAgentRunOptions.cs	Adds the nullable StoreFinalFunctionResultContent run option with detailed remarks.
dotnet/tests/Microsoft.Agents.AI.UnitTests/ChatClient/ChatClientAgentRunOptionsTests.cs	Ensures the new run option is cloned correctly.
dotnet/tests/Microsoft.Agents.AI.UnitTests/ChatClient/ChatClientAgent_StoreFinalFunctionResultContentTests.cs	Adds coverage for persistence trimming behavior (incl. streaming).