VS Code Copilot Chat 400 invalid_request_body: Invalid input[n].id 'thinking_0' (expected 'rs...')

## Summary
Copilot Chat in VS Code failed with a 400 invalid_request_body error indicating malformed input object IDs in the request payload.

## Exact Error
Sorry, your request failed. Please try again.
Client Request Id: 328d73d9-ff78-474c-81e7-2b216032bbeb
GH Request Id: F0D9:9C4A5:2792B94:29627F3:6A26DA23
Reason: Request Failed: 400 {"error":{"message":"Invalid 'input[131].id': 'thinking_0'. Expected an ID that begins with 'rs'.","code":"invalid_request_body"}}

## Environment
- OS: Windows 11
- VS Code workspace path: c:/Users/eak/Documents/Documents/Lets_Go
- Date observed: 2026-06-08
- Repo context while using Copilot Chat:
- ekimminau/Lets_Go
- branch: admin_context_enhancements

## Repro
1. Open an existing Copilot Chat session with accumulated context.
2. Submit a new prompt.
3. Request fails with 400 invalid_request_body and invalid input ID format (`thinking_0`).

## Expected
Request should be accepted and processed by Copilot Chat.

## Actual
Backend rejects request body schema due to malformed input ID format.

## Additional context
- This appears to be client-side/session-state payload assembly.
- Starting a new chat session may temporarily avoid the failure.
- Local debug logs available at:
c:/Users/eak/AppData/Roaming/Code/User/workspaceStorage/3a8678e49171e3218fc3d5fc9028b367/GitHub.copilot-chat/debug-logs/4c7041e4-89f5-4800-890d-8b2d3d465b1a

Please use the request IDs for backend/client trace correlation.