GitHub Copilot Agent fails with 408 user_request_timeout across all models even with MCP disabled

## Description

GitHub Copilot Agent is currently unusable for me because almost every request fails with HTTP 408 `user_request_timeout`.

The problem occurs across multiple models. I initially suspected that an MCP server (Figma) might be contributing to the request size, but I disabled Figma/MCP and reproduced the same issue.

Even a simple request such as:

> help me to understand this workspace?

fails with the same error.

Changing the selected model does not resolve the problem.

## Error

```text
Sorry, your request failed. Please try again.

Client Request Id: e466c439-b494-4d66-afc7-4cf25d6aef57

GH Request Id: 05F6:3EC5D3:23EF31A:293DF2B:6A9A51C6

Reason: Request Failed: 408
{"error":{"message":"Timed out reading request body. Try again, or use a smaller request size.","code":"user_request_timeout"}}