Studio: Limit depth on mu-plugins for selecting local tree

[katinthehatsite]

Related issues

Closes STU-969

Proposed Changes

This PR ensures that we limit depth on folders for mu-plugins when selecting items for push. This will ensure that it works consistently with themes and plugins.

Testing Instructions

• Pull the changes from this branch
• Start the app with npm start
• In your Studio site, add a custom folder with a plugin to mu-plugins
• Navigate to the Sync tab
• Connect that Studio site to one of the WP.com sites
• Click on Push option to open the Sync dialog`
• Confirm that you can select the plugin folders for mu-plugins but you can't go deeper than that and that the behaviour is consistent with plugins and themes:

Pre-merge Checklist

• Have you checked for TypeScript, React or other console errors?

[ivan-ottinger]

The change looks good and works as expected. 👌🏼

Before:

After:

Left just one minor comment.

[github-actions]

📊 Performance Test Results

Comparing 35afed1 vs trunk

site-editor

Metric	trunk	35afed1	Diff	Change
load	7293.00 ms	8097.00 ms	+804.00 ms	🔴 11.0%

site-startup

Metric	trunk	35afed1	Diff	Change
siteCreation	11955.00 ms	12009.00 ms	+54.00 ms	🔴 0.5%
siteStartup	3917.00 ms	3964.00 ms	+47.00 ms	🔴 1.2%

---

Results are median values from multiple test runs.

Legend: 🟢 Improvement (faster) | 🔴 Regression (slower) | ⚪ No change

[epeicher]

Thanks for solving this. I agree that this should be consistent with plugins and themes. I have tested it, and I can see only one level more as expected. LGTM!

Before	After