.NET: Populate AgentResponse metadata in CopilotStudioAgent

[anneheartrecord]

Closes #6790.

CopilotStudioAgent only set AgentId/ResponseId on the non-streaming AgentResponse, leaving CreatedAt, FinishReason, RawRepresentation and AdditionalProperties unset; the streaming path was also missing CreatedAt and FinishReason. This brings the provider in line with the others (e.g. A2AAgent) and clears the existing TODOs in the source.

Changes:

• ActivityProcessor maps the activity Timestamp to ChatMessage.CreatedAt and copies IActivity.Properties into ChatMessage.AdditionalProperties (left null when there are none).
• Non-streaming RunAsync populates CreatedAt, FinishReason (Stop), RawRepresentation and AdditionalProperties from the final message.
• Streaming RunStreamingAsync carries CreatedAt on each update and sets FinishReason only on the terminal update. It uses one-message lookahead but still emits already-received content and rethrows if the underlying stream faults, so the prior streaming behavior is preserved on the error path.
• Added unit tests covering the activity-to-message mapping, response-level metadata, the terminal finish reason, single-message streams, and the mid-stream fault case. ActivityProcessor is exposed to the test project via InternalsVisibleTo.

Usage is left unset since the Copilot Studio client does not currently surface token usage.

[Copilot]

Pull request overview

This PR updates the .NET Copilot Studio provider to populate AgentResponse / AgentResponseUpdate metadata consistently with other agent implementations, and adds unit tests to validate the new mappings.

Changes:

• Map Copilot Studio activity Timestamp to ChatMessage.CreatedAt and propagate activity Properties to ChatMessage.AdditionalProperties (null when empty).
• Populate non-streaming AgentResponse metadata (CreatedAt, FinishReason, RawRepresentation, AdditionalProperties) from the final message.
• Update streaming projection to carry CreatedAt on each update and set FinishReason only on the terminal update, plus add unit tests and expose internals to the test assembly.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 3 comments.

File	Description
dotnet/tests/Microsoft.Agents.AI.UnitTests/CopilotStudioAgentTests.cs	Adds unit tests covering metadata mapping for activities, responses, and streaming updates (including error propagation).
dotnet/src/Microsoft.Agents.AI.CopilotStudio/Microsoft.Agents.AI.CopilotStudio.csproj	Exposes internals to Microsoft.Agents.AI.UnitTests for testing ActivityProcessor and helpers.
dotnet/src/Microsoft.Agents.AI.CopilotStudio/CopilotStudioAgent.cs	Centralizes response/update creation and populates response-level metadata; refactors streaming updates to set terminal FinishReason.
dotnet/src/Microsoft.Agents.AI.CopilotStudio/ActivityProcessor.cs	Maps activity timestamp and activity properties into ChatMessage metadata.

[anneheartrecord]

@giles17 since #6790 is assigned to you — this PR fills in the missing AgentResponse metadata (CreatedAt, FinishReason, RawRepresentation, AdditionalProperties) in CopilotStudioAgent for both the streaming and non-streaming paths. CI is green; happy to adjust if you'd rather take a different approach.

[giles17]

Hey @anneheartrecord please can you address copilot comments and CI failures?

[anneheartrecord]

@giles17 Done. Rebased onto latest main — the build failure was Microsoft.OpenApi 2.0.0 (NU1903) in an unrelated sample project, which main has already remediated by pinning 2.7.5. Addressed the Copilot comments too: guarded DisposeAsync() in the streaming path so it can't mask the original stream fault, and disposed the JsonDocument in the test via using + Clone(). Left a note on the third about the streaming lookahead. Green locally — the CopilotStudio project builds with --warnaserror and all 16 CopilotStudio tests pass. Ready for another look.

[anneheartrecord]

@giles17 CI is fixed — the check-format failure was 6 async test methods in CopilotStudioAgentTests.cs missing the Async suffix (IDE1006). Renamed them in 3a1411f; dotnet format --verify-no-changes is clean locally and all 16 CopilotStudio tests pass.

The workflow runs on the new head are sitting in action_required — could you approve them when you get a chance? After that this should be green and ready for re-review. Thanks! 🙏

[giles17]

Automated Code Review

Reviewers: 5 | Confidence: 88% | Result: All clear

Reviewed: Correctness, Security Reliability, Test Coverage, Failure Modes, Design Approach

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Automated review by giles17's agents