ci: fail fork pr if formatting is required

Author: sonus21

.github/workflows/pr-format.yaml

@@ -13,13 +13,12 @@ on:
     paths:
       - "**/*.java"
 
-permissions:
-  contents: write
-
 jobs:
   format:
     if: github.event_name == 'push' || github.event.pull_request.head.repo.full_name == github.repository
     runs-on: ubuntu-latest
+    permissions:
+      contents: write
 
     steps:
       - name: Checkout branch
@@ -59,3 +58,40 @@ jobs:
           git add -A
           git commit -m "Apply Palantir Java Format"
           git push
+
+  check-format:
+    if: github.event_name == 'pull_request' && github.event.pull_request.head.repo.full_name != github.repository
+    runs-on: ubuntu-latest
+    permissions:
+      contents: read
+
+    steps:
+      - name: Checkout branch
+        uses: actions/checkout@v4
+
+      - name: Set up Java 21
+        uses: actions/setup-java@v4
+        with:
+          distribution: temurin
+          java-version: "21"
+          cache: gradle
+
+      - name: Expose Java 21 to Gradle toolchains
+        run: |
+          echo "JAVA_HOME=$JAVA_HOME" >> "$GITHUB_ENV"
+          echo "ORG_GRADLE_JAVA_INSTALLATIONS_PATHS=$JAVA_HOME" >> "$GITHUB_ENV"
+          java -version
+          javac -version
+
+      - name: Set up Gradle
+        uses: gradle/actions/setup-gradle@v4
+
+      - name: Check Java formatting
+        run: ./gradlew checkFormatJava
+
+      - name: Explain how to fix formatting
+        if: failure()
+        run: |
+          echo "Java formatting check failed."
+          echo "This pull request comes from a fork, so CI cannot push formatting commits back to the branch."
+          echo "Run './gradlew formatJava' locally and push the formatted changes to update the PR."