ci: add tests and update readme

Author: Qbandev

.github/workflows/build-lint-test.yml

@@ -253,3 +253,35 @@ jobs:
       - name: Make sure the Git HEAD did not change
         if: ${{ env.NEW_SHA != env.INITIAL_SHA }}
         run: exit 1
+
+  test-custom-yarn-url:
+    name: Test custom Yarn URL
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v4
+      - name: Checkout and setup environment with custom Yarn URL
+        id: checkout-and-setup
+        uses: ./
+        with:
+          is-high-risk-environment: false
+          yarn-custom-url: 'https://example.com/yarn-4.9.1/yarn.js#sha224.dummyhash'
+        continue-on-error: true # The URL is dummy, so this is just to exercise the code path
+      - name: Ensure custom Yarn logic was attempted
+        run: |
+          grep 'Preparing and activating custom Yarn from URL' $GITHUB_STEP_SUMMARY || exit 1
+
+  test-yarn-install-max-retries:
+    name: Test yarn-install-max-retries
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v4
+      - name: Checkout and setup environment with custom max retries
+        id: checkout-and-setup
+        uses: ./
+        with:
+          is-high-risk-environment: false
+          yarn-install-max-retries: '2'
+        continue-on-error: true # The install will fail if no yarn.lock, but we want to check retry logic
+      - name: Check retry count in logs
+        run: |
+          grep -c 'yarn --immutable' $GITHUB_STEP_SUMMARY | grep 2 || exit 1

README.md

@@ -34,6 +34,17 @@ This TypeScript module is maintained in the style of the MetaMask team.
     fetch-depth: 1
 ```
 
+#### Custom Yarn URL and Install Retries
+
+```yaml
+- name: Checkout and setup with custom Yarn
+  uses: MetaMask/action-checkout-and-setup@v1
+  with:
+    is-high-risk-environment: false
+    yarn-custom-url: 'https://your-cdn.com/yarn-4.9.1/yarn.js#sha224.abc123'
+    yarn-install-max-retries: '7'
+```
+
 ### Options
 
 #### `is-high-risk-environment`
@@ -77,6 +88,18 @@ If set to `true`, the action will skip the `yarn allow-scripts` step. This can s
 
 Defaults to `false`.
 
+#### `yarn-custom-url`
+
+If set, provides a custom URL for a Yarn bundle to be prepared and activated by Corepack. This is useful for CI environments that need to use a self-hosted or alternative Yarn bundle (for example, to avoid rate-limiting from the public Yarn registry).
+
+Defaults to `''` (not set).
+
+#### `yarn-install-max-retries`
+
+Sets the maximum number of retries for the `yarn --immutable` install command. This helps handle transient network errors more gracefully in CI environments.
+
+Defaults to `5`.
+
 ## Contributing
 
 ### Setup