fix(tui): allow (@ to trigger file selector in prompt

[MaxLeiter]

Issue for this PR

Closes #17129

Type of change

• Bug fix
• New feature
• Refactor / code improvement
• Documentation

What does this PR do?

• Allow (@ to trigger the file selector popup in the TUI, so you can write things like See this file (@path/to/file)
• Only triggers when ( is preceded by whitespace or is at the start of input — foo(@ does not trigger
• Skips the trailing space after file selection when triggered via (@

How did you verify your code works?

Manually tested the dev build

Screenshots / recordings

CleanShot.2026-03-11.at.22.47.52.mp4

Checklist

• I have tested my changes locally
• I have not included unrelated changes in this PR

If you do not follow this template your PR will be automatically rejected.

[github-actions]

Thanks for your contribution!

This PR doesn't have a linked issue. All PRs must reference an existing issue.

Please:

1. Open an issue describing the bug/feature (if one doesn't exist)
2. Add Fixes #<number> or Closes #<number> to this PR description

See CONTRIBUTING.md for details.

[github-actions]

This PR doesn't fully meet our contributing guidelines and PR template.

What needs to be fixed:

• No issue referenced. Please add Closes #<number> linking to the relevant issue.

Please edit this PR description to address the above within 2 hours, or it will be automatically closed.

If you believe this was flagged incorrectly, please let a maintainer know.

[github-actions]

This pull request has been automatically closed because it was not updated to meet our contributing guidelines within the 2-hour window.

Feel free to open a new pull request that follows our guidelines.