Pull Request for Issue #46284.

Summary of Changes

At the end of the installation process, Joomla attempts to delete the installation folder.
On Windows, this operation consistently throws a FilesystemException because PHP locks the currently executing index.php file, preventing its immediate removal.

The exception was not caught, causing a fatal error and breaking the cleanup process.

This PR fixes the issue by wrapping the folder deletion in a try/catch block and returning false when an exception is thrown, instead of letting it crash.

Additionally, a Windows-specific workaround has been added:
if the installation folder contains no subfolders and only the (already deleted but still locked) index.php file, we can assume the deletion was effectively successful and continues the cleanup process.

Testing Instructions

• You can only see the issue and test this PR on Windows
• Follow instructions at #46284, confirm the issue
• You cannot test this using Patch Tester, unfortunately. You can test it by download Joomla 5.4 from https://downloads.joomla.org/cms/joomla5/, then replace the file installation/src/Model/CleanupModel.php with the modified file in this PR, then run the installation and make sure the error is gone.

Actual result BEFORE applying this Pull Request

• Error while installing Joomla on Windows

Expected result AFTER applying this Pull Request

• The error is fixed

Link to documentations

Please select:

• Documentation link for docs.joomla.org:

• No documentation changes for docs.joomla.org needed

• Pull Request link for manual.joomla.org:

• No documentation changes for manual.joomla.org needed

PS: I used ChatGTP to improve the PR description and code comment to make it more clear :D. It provides better, clear messages than mine :D