ci: add dedicated RAT license check workflow for all PRs (#3664)

* ci: add dedicated RAT license check workflow for all PRs

The existing build workflows (pr_build_linux, spark_sql_test, etc.)
use paths-ignore to skip docs-only changes, which also skips the RAT
license header check. This allowed files without Apache license headers
to be merged via docs-only PRs.

Add a lightweight dedicated workflow that runs RAT on all PRs with no
path filters. Uses -N (non-recursive) to skip child module builds since
RAT scans the full directory tree from the parent pom.

* fix: add explicit permissions to RAT check workflow

* ci: use ubuntu-slim runner for RAT check

Author: andygrove

.github/workflows/pr_rat_check.yml

@@ -0,0 +1,47 @@
+# Licensed to the Apache Software Foundation (ASF) under one
+# or more contributor license agreements.  See the NOTICE file
+# distributed with this work for additional information
+# regarding copyright ownership.  The ASF licenses this file
+# to you under the Apache License, Version 2.0 (the
+# "License"); you may not use this file except in compliance
+# with the License.  You may obtain a copy of the License at
+#
+#   http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing,
+# software distributed under the License is distributed on an
+# "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
+# KIND, either express or implied.  See the License for the
+# specific language governing permissions and limitations
+# under the License.
+
+name: RAT License Check
+
+concurrency:
+  group: ${{ github.repository }}-${{ github.head_ref || github.sha }}-${{ github.workflow }}
+  cancel-in-progress: true
+
+permissions:
+  contents: read
+
+# No paths-ignore: this workflow must run for ALL changes including docs
+on:
+  push:
+    branches:
+      - main
+  pull_request:
+  workflow_dispatch:
+
+jobs:
+  rat-check:
+    name: RAT License Check
+    runs-on: ubuntu-slim
+    steps:
+      - uses: actions/checkout@v4
+      - name: Set up Java
+        uses: actions/setup-java@v4
+        with:
+          distribution: temurin
+          java-version: 11
+      - name: Run RAT check
+        run: ./mvnw -B -N apache-rat:check