Migrate from VS2026 Preview Scout to production VS2026 build image

[steveisok]

We currently use the VS2026 preview scout image (windows.vs2026preview.scout.amd64) across our Windows build pools. Once the production VS2026 image becomes available, we should migrate to it for stability and long-term support.

Likely files to update:

eng/pipelines/common/xplat-setup.yml - Official and public Windows build pools
eng/pipelines/libraries/fuzzing/deploy-to-onefuzz.yml - Fuzzing job pool
eng/pipelines/libraries/stress/ssl.yml - SSL stress test pool
eng/pipelines/libraries/stress/http.yml - HTTP stress test pool

Expected changes:

windows.vs2026preview.scout.amd64 → windows.vs2026.amd64
windows.vs2026preview.scout.amd64.open → windows.vs2026.amd64.open

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[steveisok]

#123561 is the VMR counterpart.