Fix unhandled PermissionError during file retention sweep on Windows

[aryansk]

Fixes #1495

Problem

On program exit, loguru deletes log files that fall out of a retention policy. On Windows, when another process still holds one of those files open, os.remove() raises PermissionError, which:

1. propagates unhandled during interpreter exit (stack trace), and
2. aborts the sweep, so the remaining out-of-retention files that could have been deleted are left behind.

The same class of failure occurs when a file is deleted concurrently between the glob() and the os.remove() call (FileNotFoundError).

Change

Both built-in retention functions (Retention.retention_count and Retention.retention_age) now handle per-file PermissionError and FileNotFoundError — skipping the affected file and continuing the sweep, so the failure of one file never blocks the deletion of the others or bubbles up during exit. retention_count's sort key also tolerates a file that disappears before sorting.

The behavior of user-supplied retention= functions is unchanged (exceptions from custom functions still propagate, as covered by the existing test_exception_during_retention_at_remove).

Testing

• New regression tests: locked file skipped while others are still deleted (count-based), concurrently-deleted file tolerated (count-based), locked file skipped (age-based). All three fail on main and pass with the fix.
• Full suite: 1595 passed, 53 skipped.

Note on AI-assisted contribution, per loguru's CONTRIBUTING policy: this change was drafted with the assistance of an AI coding tool and reviewed and tested by a human before submission.