Fix OpenAI responses streaming to preserve encrypted reasoning content

[stephentoub]

Microsoft Reviewers: Open in CodeFlow

[Copilot]

Pull request overview

This pull request fixes a bug in OpenAI streaming responses where encrypted reasoning content was being lost during the streaming process. The fix ensures that when a ReasoningResponseItem contains encrypted content in the output_item.done event, that encrypted content is properly preserved and coalesced with the streamed text deltas, enabling correct roundtripping of encrypted reasoning data.

Changes:

• Added streaming support for encrypted reasoning content by yielding a TextReasoningContent with only ProtectedData when encrypted content is present
• Added comprehensive unit test for streaming encrypted content preservation
• Added integration tests for roundtripping encrypted content in both streaming and non-streaming scenarios

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 1 comment.

File	Description
src/Libraries/Microsoft.Extensions.AI.OpenAI/OpenAIResponsesChatClient.cs	Added new case in streaming handler to yield encrypted content from ReasoningResponseItem when present, ensuring it gets coalesced with text deltas
test/Libraries/Microsoft.Extensions.AI.OpenAI.Tests/OpenAIResponseClientTests.cs	Added unit test verifying that encrypted content is preserved through streaming and coalescing
test/Libraries/Microsoft.Extensions.AI.OpenAI.Tests/OpenAIResponseClientIntegrationTests.cs	Added integration tests for both streaming and non-streaming roundtrip scenarios with encrypted content