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_

まず、Copilot CLI が使用する組み込みの grep ツールを特定し、issue に記載されている大規模なモノレポでの検索を再現します。その動作を ripgrep と比較し、必要に応じて部分的な結果も含め、検索が妥当な時間内に結果を返すことを確認します。

索引モデルが issue の本文から書いたものです。