🐛 fix(execute): keep spaces in quoted shebang interpreter paths - #4064

shebang() used str.split() on the rest of the #! line. A quoted interpreter path that contains spaces was therefore broken into several argv entries, so TOX_LIMITED_SHEBANG rewrote the invocation to a command that does not exist.

#! "/Program Files/Python/python.exe" -c
split()      -> ['"/Program', 'Files/Python/python.exe"', '-c']
shlex.split  -> ['/Program Files/Python/python.exe', '-c']

Unquoted lines (#! /bin/python \t-c\t) stay the same. An unbalanced quote is treated as unparseable, matching the existing non-UTF-8 path.

The added test fails on the unpatched tree and passes with the change.