Fix: recognise Windows drive-letter absolute paths in FileFinder

[joshdaugherty]

What:

• Bug Fix
• New Feature

Description:

On Windows, running pest --mutate without an explicit --path finds nothing to mutate — it prints 0 Mutations for 0 Files created, even with a coverage driver installed.

When you don't pass --path, the plugin takes its directories from PHPUnit's <source> configuration, and PHPUnit gives those as absolute paths. FileFinder::separateDirectoriesAndFiles() then decides whether each path is relative like this:

if (! str_starts_with($path, DIRECTORY_SEPARATOR)) {
    $path = getcwd().DIRECTORY_SEPARATOR.$path;
}

On Windows, DIRECTORY_SEPARATOR is \, but an absolute Windows path begins with a drive letter such as C:\project\app — not \. So this treats the path as relative and adds getcwd() in front of it, producing a path that doesn't exist: C:\project\C:\project\app. is_dir() is then false, the directory is skipped, and the finder has nothing to scan.

This only happens on Windows. On Linux and macOS an absolute path starts with /, which is DIRECTORY_SEPARATOR, so the check works and CI never sees it. Passing a relative --path=app also avoids it, which is the usual workaround.

The fix. Add a small isAbsolutePath() check that also treats a drive-letter path (C:\... or C:/...) and a UNC path (\\server\share) as absolute — the same way PHPUnit decides this itself. Relative paths still get getcwd() added in front, so --path=app and similar keep working, and nothing changes on Linux or macOS.

Tests. A new tests/Unit/Support/FileFinderTest.php:

• checks the new method directly for absolute paths (Unix /..., UNC, and Windows drive letters in upper and lower case) and for relative paths (including C:foo, which is relative, and an empty string), so the Windows cases are covered even when the suite runs on Linux;
• runs FileFinder::files() against a real absolute source directory and confirms it finds the fixture files. This fails on the current 4.x on Windows (it finds nothing) and passes with the fix.

Notes.

• Nothing changes on Linux or macOS: absolute paths still start with /, and relative paths are still resolved against getcwd().
• The same str_starts_with($x, DIRECTORY_SEPARATOR) check is also used in FileFinder::buildPathsToIgnore() (lines 63 and 69 on 4.x), so a Windows drive-letter path in the ignore list has the same problem and may not be excluded. That is less serious than the source-path case — a missed exclusion rather than zero mutations — has no reproduction yet, and fixing it properly also means reworking the nearby getcwd() handling, so I left it out to keep this PR focused. Flagging it here; happy to follow up.
• This edits the same line as Fix: FileFinder silently drops glob paths from phpunit source directories #23, which fixes a separate glob-pattern case just below it. The two changes are independent and apply cleanly together.
• composer test:lint, test:types, test:refacto, test:type-coverage, and test:unit all pass.