Default `github.copilot.chat.responsesApiReasoningSummary` to `off` and remove the experimental setting

[jukasper]

Description

Change the default of github.copilot.chat.responsesApiReasoningSummary from detailed to off, and remove the experimental setting.

Current state

• Declared as an experiment-based setting defaulting to detailed:
  • extensions/copilot/src/platform/configuration/common/configurationService.ts

    export const ResponsesApiReasoningSummary = defineSetting<'off' | 'detailed'>('chat.responsesApiReasoningSummary', ConfigType.ExperimentBased, 'detailed');

  • extensions/copilot/package.json — "default": "detailed" with tags ["experimental", "onExp"].
• Consumed in extensions/copilot/src/platform/endpoint/node/responsesApi.ts to set reasoning.summary on Responses API requests ('off' ⇒ no summary sent).

Requested changes

• Change the default value from detailed to off.
• Remove the experimental setting (experimental / onExp treatment).