Stabilize ResponseContinuationToken / background-response APIs

[jozkee]

Microsoft Reviewers: Open in CodeFlow

[Copilot]

Pull request overview

This PR stabilizes the background-response continuation token surface area in Microsoft.Extensions.AI.Abstractions by removing the experimental gating and updating tests/metadata to reflect the now-stable API.

Changes:

• Removes Experimental annotations (and the associated diagnostic ID) from ResponseContinuationToken and background-response related properties.
• Promotes ContinuationToken / AllowBackgroundResponses properties to be the directly-serialized public properties (removing the prior *Core indirection pattern).
• Updates stabilization and unit tests to validate JSON roundtripping using AIJsonUtilities.DefaultOptions, and marks API stages as Stable in the API metadata JSON.

Reviewed changes

Copilot reviewed 9 out of 9 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
test/Libraries/Microsoft.Extensions.AI.Stabilization.Tests/Microsoft.Extensions.AI.Stabilization.Tests.csproj	Adds the continuation-token test to the stabilization test suite.
test/Libraries/Microsoft.Extensions.AI.Abstractions.Tests/ResponseContinuationTokenTests.cs	Updates JSON roundtrip test to use AIJsonUtilities.DefaultOptions for serialization/deserialization.
src/Shared/DiagnosticIds/DiagnosticIds.cs	Removes the unused experiment ID entry for response continuations.
src/Libraries/Microsoft.Extensions.AI.OpenAI/ResponsesClientContinuationToken.cs	Removes experimental annotation/imports in line with stabilization.
src/Libraries/Microsoft.Extensions.AI.Abstractions/ResponseContinuationToken.cs	Removes experimental annotation/imports to make the token type stable.
src/Libraries/Microsoft.Extensions.AI.Abstractions/Microsoft.Extensions.AI.Abstractions.json	Marks ResponseContinuationToken and related background-response members as Stable.
src/Libraries/Microsoft.Extensions.AI.Abstractions/ChatCompletion/ChatResponseUpdate.cs	Makes ContinuationToken the public JSON-included property (removes prior wrapper + core split).
src/Libraries/Microsoft.Extensions.AI.Abstractions/ChatCompletion/ChatResponse.cs	Same stabilization pattern for ChatResponse.ContinuationToken.
src/Libraries/Microsoft.Extensions.AI.Abstractions/ChatCompletion/ChatOptions.cs	Same stabilization pattern for ChatOptions.AllowBackgroundResponses and ChatOptions.ContinuationToken.

[jozkee]

The canonical way to drive a background response is still:

var chatOptions = new ChatOptions { AllowBackgroundResponses = true };

// Get initial response with continuation token
var response = await ChatClient.GetResponseAsync("What's the biggest animal?", chatOptions);

// Continue to poll until we get the final response
while (response.ContinuationToken is not null)
{
   chatOptions.ContinuationToken = response.ContinuationToken;
   response = await ChatClient.GetResponseAsync([], chatOptions);
   await Task.Delay(1000);
}

Anything other than [] throws InvalidOperationException "Messages are not allowed when continuing a background response using a continuation token.". So, every consumer has to know the empty-list convention.
Worth considering a small extension pair to express the intent better:

 public static Task<ChatResponse> ContinueResponseAsync(
     this IChatClient client, ChatOptions options, CancellationToken cancellationToken = default);
 public static IAsyncEnumerable<ChatResponseUpdate> ContinueStreamingResponseAsync(
     this IChatClient client, ChatOptions options, CancellationToken cancellationToken = default);

As of today, only OpenAI Responses honors continuation, so it's a small surface bloat for a single-provider feature.
Also, there's currently no way to cancel the response.
Thoughts?

[jozkee]

@SergeyMenshykh thanks, I agree with adding them at a later stage if necessary.

[tarekgh]

LGTM!