test(tools): fix Windows compat in test_search_hidden_dirs

[Thorx373]

Problem

tests/tools/test_search_hidden_dirs.py fails at collection on Windows with FileNotFoundError: [WinError 2], aborting the pytest run. Once that's resolved, two further tests fail on Windows because they shell out to Unix-only find/grep.

Root Cause

1. Two @pytest.mark.skipif decorators ran subprocess.run(["which", "rg"], ...) at collection time. which doesn't exist on Windows (it uses where), so the call raised FileNotFoundError during import, before any test ran.
2. test_find_still_returns_visible_files and test_grep_still_finds_visible_content invoke Unix find/grep directly. On Windows, find.exe is a different command (FIND: Parameter format not correct) and grep isn't present.

Fix

1. Replace the subprocess.run(["which", "rg"]) checks with cross-platform shutil.which("rg") is None. The ripgrep tests now run and pass on Windows.
2. Mark the two find/grep tests skipif(sys.platform == "win32") — they depend on Unix coreutils with no Windows equivalent, so a platform skip is correct rather than a rewrite. Behavior on macOS/Linux/CI is unchanged.

Testing

Before: FileNotFoundError at collection (entire file errors).
After: file collects; ripgrep tests pass, Unix-coreutils tests skip on Windows.

   pytest tests/tools/test_search_hidden_dirs.py -v
   7 passed, 2 skipped

Tested on Windows 11, Python 3.11.9.