In Windows, os.path.ismount() returns True for nonsense UNC shares

Bug report

Bug description:

Examples below:

• \\192.168.10.2\share1 is a valid UNC.
• \\192.168.10.34534543.2\share1 is an invalid UNC as it has five IPv4 octets and an octet is out of range.
• \\"!£$£"$%\share1 is an invalid UNC as it contains illegal characters.

C:\Users\earpt\source\repos\cowgoesmoo69\cpython>python.bat
Running Debug|x64 interpreter...
Python 3.15.0a0 (heads/main:1c598e0436, Oct 10 2025, 18:08:52) [MSC v.1944 64 bit (AMD64)] on win32
Type "help", "copyright", "credits" or "license" for more information.
>>> import os
>>> os.path.ismount("\\\\192.168.10.2\\share1")
True
>>> os.path.ismount("\\\\192.168.10.34534543.2\\share1")
True
>>> os.path.ismount("\\\\\"!£$£\"$%\\share1")
True
>>>

It appears there is no validation in ntpath.ismount() of a path that appears to be a UNC.

CPython versions tested on:

CPython main branch

Operating systems tested on:

Windows

Start with ntpath.ismount() and reproduce the UNC examples on Windows using the commands in the issue. Done means valid UNC shares retain the expected result while malformed addresses and illegal-character shares no longer return True.