.NET: Add FinishReason to AgentResponses

[westey-m]

Motivation and Context

It's currently difficult to determine when a ChatClientAgent completed because of content filtering, so adding FinishReason to AgentRunResponse, and mapping where we have enough data to do so.

#4556

Description

• Add FinishReason to AgentResponse and AgentResponseUpdate

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

Adds FinishReason to the .NET AgentResponse / AgentResponseUpdate abstractions so callers can detect non-normal completions (e.g., content filtering), and wires that value through key adapters/mergers.

Changes:

• Added ChatFinishReason? FinishReason to AgentResponse and AgentResponseUpdate, including mapping to/from ChatResponse / ChatResponseUpdate.
• Propagated finish reason through workflow merging and A2A agent conversions.
• Updated/added unit tests validating defaults and propagation behavior.

Reviewed changes

Copilot reviewed 12 out of 12 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
dotnet/src/Microsoft.Agents.AI.Abstractions/AgentResponse.cs	Adds FinishReason property; maps from ChatResponse; includes in ToAgentResponseUpdates().
dotnet/src/Microsoft.Agents.AI.Abstractions/AgentResponseUpdate.cs	Adds FinishReason property; maps from ChatResponseUpdate.
dotnet/src/Microsoft.Agents.AI.Abstractions/AgentResponseExtensions.cs	Ensures FinishReason round-trips when converting to ChatResponse/ChatResponseUpdate.
dotnet/src/Microsoft.Agents.AI.Workflows/MessageMerger.cs	Carries FinishReason into merged workflow response.
dotnet/src/Microsoft.Agents.AI.A2A/A2AAgent.cs	Sets/mapps FinishReason for A2A message/task responses and streaming updates.
dotnet/src/Microsoft.Agents.AI.Hosting.OpenAI/ChatCompletions/AgentResponseExtensions.cs	Uses AgentResponse.FinishReason when producing OpenAI-style choices.
dotnet/src/Microsoft.Agents.AI.Hosting.OpenAI/ChatCompletions/AIAgentChatCompletionsProcessor.cs	Uses AgentResponseUpdate.FinishReason for streaming chunk finish reason.
dotnet/tests/Microsoft.Agents.AI.Workflows.UnitTests/MessageMergerTests.cs	Adds coverage for merged FinishReason behavior.
dotnet/tests/Microsoft.Agents.AI.Abstractions.UnitTests/AgentResponseUpdateTests.cs	Verifies default/null and round-tripping for AgentResponseUpdate.FinishReason.
dotnet/tests/Microsoft.Agents.AI.Abstractions.UnitTests/AgentResponseUpdateExtensionsTests.cs	Verifies conversion helpers preserve FinishReason.
dotnet/tests/Microsoft.Agents.AI.Abstractions.UnitTests/AgentResponseTests.cs	Verifies default/null and round-tripping for AgentResponse.FinishReason + update generation.
dotnet/tests/Microsoft.Agents.AI.A2A.UnitTests/A2AAgentTests.cs	Updates assertions to validate FinishReason for A2A runs/streams.