Fix #19423: Add VSCMD_VER guard to conanvcvars.bat

[anushkagupta200615-jpg]

Changelog: Fix: VCVars generator now skips calling vcvarsall.bat if it is already active to prevent %PATH% length crashes on repeated activations.
Docs: N/A

• Refer to the issue that supports this Pull Request.
  Fixes [bug] conanvcvars.bat should activate only once, since it can't be deactivated. #19423
• If the issue has missing info, explain the purpose/use case/pain/need that covers this Pull Request.
  The generated conanvcvars.bat blindly called Microsoft's vcvarsall.bat every time conanbuild.bat was executed. Because Microsoft's scripts are not idempotent and have no real deactivation mechanism, repeated activations appended duplicate entries to %PATH%. This eventually caused the environment to hit the Windows character limit and crash. The generated scripts (.bat and .ps1) now include a guard that checks VSCMD_VER to skip activation if the Visual Studio developer environment is already active.
• I've read the Contributing guide.
• I've followed the PEP8 style guides for Python code.
• I've opened another PR in the Conan docs repo to the develop branch, documenting this one.

[memsharded]

Hi @anushkagupta200615-jpg

Thanks for your contribution.
This would be a duplicate of #19424

I did a review there and shared some concerns in #19424 (review). I think this change can be risky and break some users.

I think the conversation should be followed up in that PR, not open a new one, I suggest closing this one and commenting in that other PR.