grep tool times out on large repos before returning any results

### Describe the bug

When using the built in grep tool in a large mono repo, the tool returns "timeout" with no results. I had to instruct copilot to use ripgrep rg, instead.

### Affected version

GitHub Copilot CLI 1.0.36

### Steps to reproduce the behavior

1. Open Copilot CLI in a large monorepo
2. Ask to search for a pattern across a folder and its subdirectories, dont nudge it to use ripgrep yet
3. It'll start grepping and maybe timeout

### Expected behavior

The grep tool should return results (even partial) within a reasonable time

### Additional context

_No response_

Start by locating the built-in grep tool used by Copilot CLI and reproduce the search across a large monorepo described in the issue. Compare its behavior with ripgrep and verify that searches return results, including partial results where appropriate, within a reasonable time.

Written by the indexing model from the issue text.