Fix OpenAIResponsesChatClient to respect "store":false in responses

[stephentoub]

Fixes #7406

Microsoft Reviewers: Open in CodeFlow

[Copilot]

Pull request overview

This PR updates OpenAIResponsesChatClient so that ConversationId is set to null when the actual OpenAI Responses API result indicates "store": false, even if the request options didn’t explicitly disable stored output. This aligns the abstraction with provider behavior (e.g., OpenRouter) and prevents consumers from assuming a resumable/stored conversation when it isn’t.

Changes:

• Update non-streaming response conversion to null out ConversationId when the response’s "store" field is false.
• Update streaming response processing to track "store": false across events and consistently emit ConversationId = null.
• Add test coverage for both non-streaming and streaming cases where the response (not the request) contains "store": false.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
src/Libraries/Microsoft.Extensions.AI.OpenAI/OpenAIResponsesChatClient.cs	Derives ConversationId based on both request options and the response’s "store" field (via Patch).
test/Libraries/Microsoft.Extensions.AI.OpenAI.Tests/OpenAIResponseClientTests.cs	Adds regression tests ensuring ConversationId is null when the response includes "store": false (streaming + non-streaming).