msvc: VS2026: unlock picky warning in cmake, test in CI

[vszakats]

Upgrade a GHA/windows job to VS2026 (from VS2022), using a runner image
released a week ago. It also comes with the same Windows SDK as VS2022:
v10.0.26100.0.

The runner image uses Windows 2025 unfortunately, which makes the job
run significantly slower than before this patch:

• configure: 49s → 1m10s
• build: 3s → 5s
• install test prereqs: 23s → 27s
• run tests: 3m18s → 4m11s
• build examples: 15s → 25s

It's a shame.

Also:

• cmake: enable picky warnings for VS2026 internal version 19.50.
  Build is clean with existing options.
• GHA/windows: make the built-in OpenSSH intall path recognize
  the windows-2025-vs2026 image as windows-2025.
• windows-2025-vs2026 is able to load the cached stunnel made on
  the windows-2022 runner.
• disk use of the build is almost identical to VS2022.

Before: https://github.com/curl/curl/actions/runs/21955482367/job/63418133880
After: https://github.com/curl/curl/actions/runs/21957589847/job/63426546943

Ref: https://github.com/actions/runner-images/blob/71f0157880be328a2b0a4dc81cda61735ba6c45d/images/windows/Windows2025-VS2026-Readme.md
Ref: #20575

[vszakats]

/usr/bin/find: ‘/c/PROGRA~1/MICROS~2/2026/Enterprise/vc/tools/msvc’: No such file or directory

of course!