ci: install Yarn Switch in CI workflows

Yarn v6 is distributed via Yarn Switch (not Corepack/npm), so CI
environments need it installed before running yarn commands.

- Add composite setup-toolchain action that wraps the upstream action
  and installs Yarn Switch, handling Linux/macOS vs Windows differences
- Update pr.yml to use the local composite action in all jobs, replacing
  the separate setup-toolchain + Install Yarn Switch steps
- Replace corepack enable with Yarn Switch in check-changesets job
- Add Yarn Switch installation to Azure Pipelines publish workflow

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: Saadnajmi

.ado/azure-pipelines.publish.yml

@@ -65,6 +65,11 @@ extends:
                   version: '22.x'
                 displayName: 'Use Node.js 22.x'
 
+              - script: |
+                  curl -s https://repo.yarnpkg.com/install | bash
+                  echo "##vso[task.prependpath]$(HOME)/.yarn/switch/bin"
+                displayName: 'Install Yarn Switch'
+
               - script: |
                   yarn
                 displayName: 'yarn install'
@@ -104,6 +109,11 @@ extends:
                   version: '22.x'
                 displayName: 'Use Node.js 22.x'
 
+              - script: |
+                  curl -s https://repo.yarnpkg.com/install | bash
+                  echo "##vso[task.prependpath]$(HOME)/.yarn/switch/bin"
+                displayName: 'Install Yarn Switch'
+
               - task: DownloadPipelineArtifact@2
                 inputs:
                   artifactName: packed-tarballs

.github/actions/setup-toolchain/action.yml

@@ -0,0 +1,40 @@
+name: Set up toolchain
+description: Sets up Node.js, platform-specific tools, and Yarn Switch
+
+inputs:
+  platform:
+    description: Target platform (android, ios, macos, windows)
+    required: false
+    default: ''
+  node-version:
+    description: Node.js version to use
+    required: false
+    default: '22'
+  xcode-developer-dir:
+    description: Path to Xcode developer directory (macOS only)
+    required: false
+    default: ''
+
+runs:
+  using: composite
+  steps:
+    - name: Set up toolchain
+      uses: microsoft/react-native-test-app/.github/actions/setup-toolchain@5.0.14
+      with:
+        platform: ${{ inputs.platform }}
+        node-version: ${{ inputs.node-version }}
+        xcode-developer-dir: ${{ inputs.xcode-developer-dir }}
+
+    - name: Install Yarn Switch
+      if: runner.os != 'Windows'
+      shell: bash
+      run: |
+        curl -s https://repo.yarnpkg.com/install | bash
+        echo "$HOME/.yarn/switch/bin" >> $GITHUB_PATH
+
+    - name: Install Yarn Switch (Windows)
+      if: runner.os == 'Windows'
+      shell: pwsh
+      run: |
+        curl -s https://repo.yarnpkg.com/install | bash
+        echo "$HOME/.yarn/switch/bin" >> $env:GITHUB_PATH

.github/workflows/pr.yml

@@ -20,7 +20,7 @@ jobs:
           fetch-depth: 0
 
       - name: Set up toolchain
-        uses: microsoft/react-native-test-app/.github/actions/setup-toolchain@5.0.14
+        uses: ./.github/actions/setup-toolchain
         with:
           node-version: 22
 
@@ -39,7 +39,7 @@ jobs:
         uses: actions/checkout@v4
 
       - name: Set up toolchain
-        uses: microsoft/react-native-test-app/.github/actions/setup-toolchain@5.0.14
+        uses: ./.github/actions/setup-toolchain
         with:
           platform: android
           node-version: 22
@@ -69,7 +69,7 @@ jobs:
         uses: actions/checkout@v4
 
       - name: Set up toolchain
-        uses: microsoft/react-native-test-app/.github/actions/setup-toolchain@5.0.14
+        uses: ./.github/actions/setup-toolchain
         with:
           platform: macos
           node-version: 22
@@ -124,7 +124,7 @@ jobs:
         uses: actions/checkout@v4
 
       - name: Set up toolchain
-        uses: microsoft/react-native-test-app/.github/actions/setup-toolchain@5.0.14
+        uses: ./.github/actions/setup-toolchain
         with:
           platform: ios
           node-version: 22
@@ -172,7 +172,7 @@ jobs:
           fetch-depth: 5
 
       - name: Set up toolchain
-        uses: microsoft/react-native-test-app/.github/actions/setup-toolchain@5.0.14
+        uses: ./.github/actions/setup-toolchain
         with:
           platform: windows
           node-version: 22
@@ -228,7 +228,7 @@ jobs:
         uses: actions/checkout@v4
 
       - name: Set up toolchain
-        uses: microsoft/react-native-test-app/.github/actions/setup-toolchain@5.0.14
+        uses: ./.github/actions/setup-toolchain
         with:
           platform: windows
           node-version: 22
@@ -280,14 +280,10 @@ jobs:
           filter: blob:none
           fetch-depth: 0
 
-      - name: Setup Node.js
-        uses: actions/setup-node@v6
+      - name: Set up toolchain
+        uses: ./.github/actions/setup-toolchain
         with:
           node-version: '22'
-          cache: 'yarn'
-
-      - name: Enable Corepack
-        run: corepack enable
 
       - name: Install dependencies
         run: yarn install --immutable
@@ -306,7 +302,7 @@ jobs:
         uses: actions/checkout@v4
 
       - name: Set up toolchain
-        uses: microsoft/react-native-test-app/.github/actions/setup-toolchain@5.0.14
+        uses: ./.github/actions/setup-toolchain
         with:
           node-version: 22
 
@@ -347,7 +343,7 @@ jobs:
         uses: actions/checkout@v4
 
       - name: Set up toolchain
-        uses: microsoft/react-native-test-app/.github/actions/setup-toolchain@5.0.14
+        uses: ./.github/actions/setup-toolchain
         with:
           node-version: 22
 
@@ -372,7 +368,7 @@ jobs:
         uses: actions/checkout@v4
 
       - name: Set up toolchain
-        uses: microsoft/react-native-test-app/.github/actions/setup-toolchain@5.0.14
+        uses: ./.github/actions/setup-toolchain
         with:
           node-version: 22