Guard Activity.Current restore with null check in OpenTelemetry streaming clients

[Copilot]

OpenTelemetryChatClient.GetStreamingResponseAsync unconditionally sets Activity.Current = activity after each yield return as a workaround for dotnet/runtime#47802. When _activitySource.HasListeners() is false, activity is null, so this destroys Activity.Current for all downstream middleware on every streaming chunk. Same class of bug fixed for FunctionInvokingChatClient in #7321.

Changes

• Guard the workaround with if (activity is not null) in all four affected streaming methods:
  • OpenTelemetryChatClient
  • OpenTelemetrySpeechToTextClient
  • OpenTelemetryTextToSpeechClient
  • OpenTelemetryHostedFileClient
• Add test Streaming_NoListeners_PreservesActivityCurrent verifying Activity.Current survives streaming when no listener is attached

// Before
yield return update;
Activity.Current = activity;

// After
yield return update;
if (activity is not null)
{
    Activity.Current = activity;
}

[Copilot]

Pull request overview

Fixes an OpenTelemetry streaming regression where Activity.Current could be unintentionally cleared (set to null) on each streamed chunk when no ActivitySource listeners are attached.

Changes:

• Guard Activity.Current = activity restoration with if (activity is not null) in streaming loops for multiple OpenTelemetry client wrappers.
• Add a unit test asserting Activity.Current is preserved during streaming when the OpenTelemetry ActivitySource has no listeners.

Reviewed changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
test/Libraries/Microsoft.Extensions.AI.Tests/ChatCompletion/OpenTelemetryChatClientTests.cs	Adds streaming test to ensure Activity.Current is preserved when no OTel listeners exist.
src/Libraries/Microsoft.Extensions.AI/ChatCompletion/OpenTelemetryChatClient.cs	Prevents clearing Activity.Current during streaming when no activity was created.
src/Libraries/Microsoft.Extensions.AI/SpeechToText/OpenTelemetrySpeechToTextClient.cs	Same null-guard for streaming activity restoration in speech-to-text.
src/Libraries/Microsoft.Extensions.AI/TextToSpeech/OpenTelemetryTextToSpeechClient.cs	Same null-guard for streaming activity restoration in text-to-speech.
src/Libraries/Microsoft.Extensions.AI/Files/OpenTelemetryHostedFileClient.cs	Same null-guard for activity restoration in async enumeration of listed files.