fix(windows): prefer PATHEXT variants over bare filename in resolver (#5364)

`find_executable` used to short-circuit on the first exists()-and-is_file
match and only consult PATHEXT if no bare file was present. That returned
the wrong file for tools whose Windows installer ships both a Unix-style
launcher and a `.bat` alongside it. The reported case is Composer, whose
setup places bare `composer` (a Unix launcher, invalid PE on Windows) next
to `composer.bat`. The bare file was resolved and handed to CreateProcessW,
which rejected it with ERROR_BAD_EXE_FORMAT.

On Windows, when the candidate has no user-supplied extension, walk PATHEXT
first and only fall back to the bare match if nothing matches. This mirrors
cmd.exe's and powershell's resolution.

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