The explorer panel's explorerCurrentDir is hardcoded to null, so
the Quick Switcher's workspace toggle was a no-op. Rather than fix
the broken explorer flow, derive the workspace from the active
tab's filePath's parent directory — the most reliable source we
have without restructuring that panel.
deriveWorkspaceDir() handles POSIX and Windows paths (including
mixed separators and trailing separators), returns null for
untitled / empty inputs so the workspace toggle silently stays
off in those cases.
10 new tests cover POSIX, Windows, mixed-separator, trailing
separator, UNC, and invalid input paths.
Amit Haridas
