🐛 fix(config): keep the leading pair of a win32 UNC path

[MohammedAlkindi]

_win32_process_path_backslash collapses any run of exactly two backslashes, treating it as the POSIX-escaped form of one literal backslash. That is right in the middle of a path, but a UNC or extended-length prefix needs both leading backslashes, so a network path in commands silently loses one.

With commands = xcopy \\server\share\file.txt . on main:

$ tox config -e py -k commands
[testenv:py]
commands = xcopy \server\share\file.txt .

\\server becomes \server. No error, and the command runs against a path that does not exist.

The fix keeps a backslash pair literal only when it starts a word and is followed by more path text. A bare trailing \\ is not a prefix and still collapses, which is what WACKY_SLASH_ARGS covers; an earlier attempt that preserved every leading pair broke three of those cases.

Three cases added, failing first (assert ['\\srv\\share'] == ['\\\\srv\\share']). After: tox config reports the path intact, and tests/config/loader/test_str_convert.py is 175 passed, 5 skipped with nothing else changing status. ruff check and ruff format --check clean.

Windows-only path, so it does not show up on Linux CI. Found by inspection, not a user report.