Move OneBranch pipelines to Windows Server 2025 / VSE 2026 image

Switch the OneBranch container image from the Server 2022 / VS 2022
image to the Server 2025 / VS Enterprise 2026 image. This brings the
LLVM/Clang toolset from 19.1.x up to 20.1.8.

Per the OneBranch "2025 Infra Support" guidance, the Server 2025 host
is selected via the structured featureFlags.WindowsHostVersion Version
field rather than a host-name string. The top-level WindowsHostVersion
pipeline variable was unreferenced and is removed.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>
Copilot-Session: c700591a-bd3e-4ae9-996e-61b14b693852

Author: Ben Hillis

.pipelines/wsl-build-nightly-onebranch.yml

@@ -9,8 +9,7 @@ schedules:
   always: true
 
 variables:
-  WindowsContainerImage: "onebranch.azurecr.io/windows/ltsc2022/vse2022:latest"
-  WindowsHostVersion: '1ESWindows2022'
+  WindowsContainerImage: "onebranch.azurecr.io/windows/ltsc2025/vse2026:latest"
 
 resources:
   repositories:
@@ -26,7 +25,8 @@ extends:
       name: "windows_undocked"
     featureFlags:
       EnableCDPxPAT: false
-      WindowsHostVersion: 1ESWindows2022
+      WindowsHostVersion:
+        Version: 2025
     globalSdl:
       credscan:
         enabled: true

.pipelines/wsl-build-pr-onebranch.yml

@@ -5,8 +5,7 @@ trigger:
     - release/*
 
 variables:
-  WindowsContainerImage: "onebranch.azurecr.io/windows/ltsc2022/vse2022:latest"
-  WindowsHostVersion: '1ESWindows2022'
+  WindowsContainerImage: "onebranch.azurecr.io/windows/ltsc2025/vse2026:latest"
 
 resources:
   repositories:
@@ -22,7 +21,8 @@ extends:
       name: "windows_undocked"
     featureFlags:
       EnableCDPxPAT: false
-      WindowsHostVersion: 1ESWindows2022
+      WindowsHostVersion:
+        Version: 2025
     globalSdl:
       suppression:
         suppressionFile: $(Build.SourcesDirectory)\.gdnsuppress

.pipelines/wsl-build-release-onebranch.yml

@@ -24,8 +24,7 @@ trigger:
     include: ['*.*.*']
 
 variables:
-  WindowsContainerImage: "onebranch.azurecr.io/windows/ltsc2022/vse2022:latest"
-  WindowsHostVersion: '1ESWindows2022'
+  WindowsContainerImage: "onebranch.azurecr.io/windows/ltsc2025/vse2026:latest"
 
 resources:
   repositories:
@@ -41,7 +40,8 @@ extends:
       name: "windows_undocked"
     featureFlags:
       EnableCDPxPAT: false
-      WindowsHostVersion: 1ESWindows2022
+      WindowsHostVersion:
+        Version: 2025
     globalSdl:
       credscan:
         enabled: true