.NET: fix: preserve agent continuation token when wrapped with UseOpenTelemetry - #8265

Motivation & Context

Wrapping an agent with UseOpenTelemetry breaks background responses. The continuation token returned from a background RunAsync is the underlying provider's raw token rather than the agent's own ChatClientAgentContinuationToken, so an opaque ToBytes/FromBytes save-and-restore followed by a resume throws:

System.ArgumentException: Failed to create ChatClientAgentContinuationToken from provided token because it is not of the correct type. (Parameter 'token')
   at Microsoft.Agents.AI.ChatClientAgentContinuationToken.FromToken(ResponseContinuationToken token)
   at Microsoft.Agents.AI.ChatClientAgent.CreateConfiguredChatOptions(AgentRunOptions runOptions)

The same flow works when OpenTelemetry is not in the pipeline. This blocks the scenario where background responses are started by one process and polled or resumed by another, since that requires persisting a serialized session and token while still emitting telemetry for logging and traceability.

OpenTelemetryAgent delegates to the inner agent through OpenTelemetryChatClient and a ForwardingChatClient, so every response makes an AgentResponse -> ChatResponse -> AgentResponse round trip. AgentResponseExtensions.AsChatResponse returns the inner agent's own ChatResponse directly whenever that is the response's raw representation, which is exactly the case for ChatClientAgent. The wrapped continuation token and AgentId live only on the AgentResponse, so both were discarded on the way back out and RunCoreAsync rebuilt a response from the provider's ChatResponse, carrying the provider's raw token. RunStreamingAsync lost them the same way through AsChatResponseUpdate.

Description & Review Guide

• What are the major changes?

  • ForwardingChatClient no longer uses AsChatResponse/AsChatResponseUpdate. It now builds the ChatResponse/ChatResponseUpdate that is handed to OpenTelemetryChatClient with the inner agent's own response as the RawRepresentation. RunCoreAsync and RunCoreStreamingAsync already looked for an AgentResponse/AgentResponseUpdate there, so they now return the inner agent's instance unchanged instead of reconstructing a lossy copy.
  • ConversationId is copied across from the inner ChatResponse/ChatResponseUpdate so the chat span keeps emitting gen_ai.conversation.id.
  • Three tests in OpenTelemetryAgentTests: continuation token preservation for RunAsync and RunStreamingAsync, and the reported scenario end to end - serialize the token with ToBytes, restore it with FromBytes, resume, and assert the chat client receives its own token back. All three fail without the source change.

• What is the impact of these changes?

  • Background responses, AgentId, and any other agent-level response state now survive a UseOpenTelemetry wrapper. Callers no longer need workarounds to reconstruct the agent token.
  • No public API change, and no behavior change for callers that do not use background responses. The fix is confined to the private ForwardingChatClient; AsChatResponse/AsChatResponseUpdate keep their documented short-circuit behavior and are untouched.

• What do you want reviewers to focus on?

  • Whether copying ConversationId from the raw representation is the right way to keep the chat span intact, or whether the span should take it from the request options instead.
  • Whether other DelegatingAIAgent implementations that perform the same round trip should share a helper rather than keeping this private to OpenTelemetryAgent.

Related Issue

Fixes #8243

Contribution Checklist

• The code builds clean without any errors or warnings
• All unit tests pass, and I have added new tests where possible
• The PR follows the Contribution Guidelines
• This PR is linked to an issue and there is no other open PR for this issue (see Related Issue above).
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