fix(deleter): handle non-directory entries in log_root

[Akasxh]

Summary

• The deleter crashed with NotADirectoryError when stray files (e.g. 2023-09-23.tar.gz) or symlinks existed in log_root, because os.listdir() was called on every entry unconditionally
• Now checks os.path.isdir() before listing contents for lock files; files and symlinks are deleted directly with os.remove()
• Minimal diff — no refactoring, no new abstractions, just the guard check

Tests

Added 4 new test cases covering the scenarios from the issue:

• test_delete_stray_file — tar.gz and other non-directory files get cleaned up
• test_delete_stray_symlink — symlinks are unlinked without following the target
• test_delete_mixed_entries — mix of segment dirs, stray files, nested dirs, hidden files
• test_delete_continues_after_error — deleter moves to next entry if one fails (e.g. permission error)

Test plan

• pytest system/loggerd/tests/test_deleter.py -v passes all existing + new tests
• Verify on device with stray files in /data/media/0/realdata/

Fixes #30102

[Akasxh]

Note: this PR takes a minimal-diff approach compared to #37642 which refactors into new helper functions. The key differences:

• Minimal changes to existing logic — no new functions, just guards around the existing loop
• Stray files cleaned first — non-directory entries are prioritized before segment dirs, since they're foreign and shouldn't be there
• 4 targeted test cases covering the exact scenarios from the original report (stray tar.gz files, symlinks, mixed entries, error continuation)