Fix Windows media source paths for Flysystem

[Ibochkarev]

What changed and why

On Windows, Flysystem object keys use /, while media source code joined and split paths with DIRECTORY_SEPARATOR (\). Nested rename dropped the parent directory and moved the object to the wrong place (often under the site core path). #15440 only cleared PHP “passed by reference” notices; path creation stayed broken (confirmed in issue comments).

This change treats Flysystem object paths as always /: sanitizePath / postfixSlash, helpers for root / join / rename, and the same join logic for create, move, list ids, and S3.

How to test

1. On Windows (or with backslash-style inputs): create a nested folder under a media source, rename it, confirm it stays under the same parent.
2. Rename a nested file the same way.
3. Create a folder/file under a nested path and move an object between folders.
4. Gate E:
   • php -l core/src/Revolution/Sources/modMediaSource.php → exit 0
   • php -l core/src/Revolution/Sources/modS3MediaSource.php → exit 0
   • composer run-script phpunit -- --filter modMediaSourceTest → OK (4 tests, 15 assertions)

Related issue(s)/PR(s)

Resolves #15438

Compatibility notes

Affects local and S3 media sources on all platforms. Behavior on Unix stays the same (/ already matched DIRECTORY_SEPARATOR). Windows file manager path joins should now match Flysystem keys. Local OS path handling in prepareSrcForThumb still uses DIRECTORY_SEPARATOR for phpThumb/file_exists checks.

Breaking change assessment

No public method signatures changed. Path strings returned from media source APIs may normalize separators to / more consistently (Flysystem-compatible). Safe for patch consumers that already treated Flysystem paths as /.

Test coverage

_build/test/Tests/Model/Sources/modMediaSourceTest.php: sanitize/postfix with \, getRenamedPath parent retention, root detection after postfixSlash('') → /, joinFilesystemPaths for move/create-style joins.

Contributors

Thanks @sergant210 for the report, @wax100 for the Windows/OpenServer reproduction, and @Mark-H for confirming #15440 did not fix path creation.

AI tool use

Cursor agent assisted implementation, tests, and review loops. Human review still required before merge.