fix(core): align Copilot response continuation - #41452

Summary

• align stateless Copilot Responses continuation with the official VS Code Copilot client
• persist the final completed reasoning item ID together with encrypted state
• omit response item IDs from stateless text and tool-call reconstruction while preserving tool call_id
• handle reasoning summary completion explicitly and keep stream lifecycles balanced
• retain previousResponseId as an opt-in provider option without enabling it

Verification

• 35 focused Core tests
• Core typecheck
• full 33-package pre-push typecheck
• live github-copilot/gpt-5.4 high reasoning run through tool execution and continuation

Stack

Targets #40997 (auth-forms) so the Copilot SDK correction can be reviewed independently.