File @Mention is very slow for a large repository with ~150k files.

### Describe the bug

For a very large repository, nearly 150,000 files, @ to mention a file is very slow. After typing @(the full exact file name) it takes over 5 seconds to show any results. This is a new fast M5 Pro Mac.

#ms-teams-fb

### Affected version

GitHub Copilot CLI 1.0.52-0.

### Steps to reproduce the behavior

Type @FileName
Results should show quickly. It can take many seconds, even with a brand new fast computer.

### Expected behavior

Type @FileName
Results should show quickly.
Bonus: Search results are not case sensitive. This would be an improvement over Claude that is case sensitive.

### Additional context

MacBook Pro, Apple M5 Pro CPU, 64GB RAM.
Repository is very large. over 145,000 files.

Start by reproducing the delay in GitHub Copilot CLI 1.0.52-0 with a repository containing about 145,000 files, using an exact file-name @mention. Trace the file-mention search entry point and measure results against the expected behavior: results should appear quickly and matching should be case-insensitive.

Scritto dal modello di indicizzazione a partire dal testo della issue.