Add tooltip to FTB filter bar

Add tooltip for file tree filter shortcut visibility

What does this MR do?

This MR adds tooltips to the file tree filter in the Source Code view to display keyboard shortcuts when hovering, maintaining consistency with the recent implementation in the Code Review section (MR !199595 (merged)).

Why was this MR needed?

Currently, tooltips showing keyboard shortcuts appear when hovering over the file tree in merge requests, but this functionality is missing in the Source Code view. This inconsistency creates a disjointed user experience across GitLab interfaces.

What are the relevant screenshots?

N/A - Implementation will match the existing behavior shown in the issue:

• Tooltips appear on hover for both open and closed file trees
• Tooltips display relevant keyboard shortcuts for file tree filtering

Implementation details

• Added tooltip component to the file tree filter in Source Code view
• Ensured tooltip content matches the existing implementation in merge requests
• Maintained consistent styling and behavior with the Code Review implementation

MR acceptance checklist

• Tests added/updated
• Documentation updated (if needed)
• Verified tooltip appears correctly on hover for both open and closed file trees
• Confirmed tooltip content matches the implementation in merge requests

Closes #561066 (closed)