refactor: simplify FileLocator::listFiles()

[paulbalandan]

Description

FileLocator::listFiles() resolved each namespace's <path> via realpath(), gated get_filenames() on is_dir(), and then guarded array_merge on a non-empty result. All three checks are redundant:

• get_filenames() (which uses RecursiveDirectoryIterator under the hood) already wraps its iterator in try { ... } catch (Throwable) { return []; } (see system/Helpers/filesystem_helper.php), so a missing or non-directory path simply yields an empty list.
• array_merge($files, []) is well-defined and returns a copy of $files, so the if ($tempFiles !== []) guard is a micro-optimization for a discovery-time path.

The loop body collapses to a single array_merge call.

Side benefit: dropping the realpath/is_dir gate also avoids a class of latent flakes where PHP's stat / realpath caches hold stale negative entries for a namespace's subdirectory on Linux PHP (which caches is_dir negatives and realpath() failures, unlike macOS PHP). PR #10141 papered over one such failure in CommandsTest with clearstatcache(true); with this change, that workaround is no longer load-bearing.

Checklist:

• Securely signed commits
• Component(s) with PHPDoc blocks, only if necessary or adds value (without duplication)
• Unit testing, with >80% coverage
• User guide updated
• Conforms to style guide

[michalsn]

Okay, that makes sense.