Plugins: Ignore dot-prefixed MU plugin files

[Sukhendu2002]

Previously, dot-prefixed MU plugin files were still detected and loaded as must-use plugins. This meant files intended to be hidden, disabled, or ignored by convention could still appear in the Must-Use plugins list and execute during bootstrap.

This PR updates MU plugin discovery to ignore dot-prefixed PHP files, such as .testmu.php.

The change applies to both:

• get_mu_plugins() — admin Must-Use plugins list.
• wp_get_mu_plugins() — runtime MU plugin loading.

Trac ticket: https://core.trac.wordpress.org/ticket/41638

Manual test:

1. Add .testmu.php and testmu.php to wp-content/mu-plugins.
2. Visit the Must-Use plugins list in wp-admin.
3. Confirm only testmu.php appears.
4. Confirm .testmu.php is not loaded at runtime.

Use of AI Tools

AI assistance: No

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[github-actions]

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