Enable persistent CoT for hidden models

[dileepyavan]

Summary

• add an experiment flag for persistent chain of thought in the Responses API
• send reasoning.context: "all_turns" for hidden model M when enabled
• cover initial, unsupported, disabled, and continued-response requests

Testing

• node node_modules/vitest/vitest.mjs --run --pool=forks src/extension/test/node/configurations.spec.ts src/platform/endpoint/node/test/responsesApi.spec.ts

[Copilot]

Pull request overview

This PR adds an experiment-gated configuration to enable persistent reasoning context (“persistent CoT”) for the Responses API, and wires it so that eligible hidden model family M requests send reasoning.context: 'all_turns' (including for continued-response requests).

Changes:

• Introduces an experiment-based setting github.copilot.chat.responsesApi.persistentCoT.enabled.
• Extends the Responses API request body creation to include reasoning.context: 'all_turns' when the experiment is enabled and the endpoint is a hidden model M family.
• Adds unit tests covering enabled/disabled/unsupported and continued-response behavior.

Show a summary per file

File	Description
extensions/copilot/src/platform/networking/common/networking.ts	Extends IEndpointBody.reasoning typing to include context for Responses API requests.
extensions/copilot/src/platform/endpoint/node/test/responsesApi.spec.ts	Adds tests validating reasoning.context behavior for initial and continued Responses API requests.
extensions/copilot/src/platform/endpoint/node/responsesApi.ts	Adds experiment-gated logic to send reasoning.context: 'all_turns' for hidden model M endpoints.
extensions/copilot/src/platform/configuration/common/configurationService.ts	Registers the new experiment-based config key chat.responsesApi.persistentCoT.enabled.
extensions/copilot/package.nls.json	Adds localized description string for the new setting.
extensions/copilot/package.json	Exposes the new configuration property github.copilot.chat.responsesApi.persistentCoT.enabled in the extension manifest.

Copilot's findings

• Files reviewed: 6/6 changed files
• Comments generated: 0