Increase slash, at-command, and path completion coverage #2019

Summary

Add coverage for completion filtering, async loading, cancellation, and acceptance behavior.

Suggested coverage

• Slash suggestion filtering and no-result behavior.
• Enter and Tab acceptance behavior.
• Escape dismissal without accepting suggestions.
• At-command debounce, cancellation, and error recovery.
• Path completion with spaces, hidden files, ignored files, or special characters where applicable.

Test approach

Use hook and component tests for most cases. Keep one or two tmux smoke tests for real keyboard behavior.