build: add runners/spark/4/ build configuration

Add the Gradle build file for the Spark 4 structured streaming runner.
The module mirrors runners/spark/3/ — it inherits the shared RDD-base
source from runners/spark/src/ via copySourceBase and adds its own
Structured Streaming implementation in src/main/java.

Key differences from the Spark 3 build:
- Uses spark4_version (4.0.2) with Scala 2.13.
- Excludes DStream-based streaming tests (Spark 4 supports only
  structured streaming batch).
- Unconditionally adds --add-opens JVM flags required by Kryo on
  Java 17 (Spark 4's minimum).
- Binds Spark driver to 127.0.0.1 for macOS compatibility.

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

Author: tkaymak

runners/spark/4/build.gradle

@@ -0,0 +1,89 @@
+/*
+ * 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.
+ */
+
+def basePath = '..'
+/* All properties required for loading the Spark build script */
+project.ext {
+  // Spark 4 version as defined in BeamModulePlugin; requires Scala 2.13 and Java 17
+  spark_version = spark4_version
+  spark_scala_version = '2.13'
+  copySourceBase = true // copy shared base into build dir; Spark 3 remains the primary dev version
+  archives_base_name = 'beam-runners-spark-4'
+}
+
+// Load the main build script which contains all build logic.
+apply from: "$basePath/spark_runner.gradle"
+
+// Force Spark to bind to 127.0.0.1 so tests pass on machines where the hostname
+// doesn't resolve to a bindable address (e.g. mac.lan in macOS VPN environments).
+// Spark 4 always requires Java 17, so unconditionally add the --add-opens flags
+// required by Kryo and other libraries that use reflection on JDK internals.
+test {
+  systemProperty "spark.driver.host", "127.0.0.1"
+  systemProperty "spark.driver.bindAddress", "127.0.0.1"
+  jvmArgs "--add-opens=java.base/sun.nio.ch=ALL-UNNAMED",
+          "--add-opens=java.base/java.nio=ALL-UNNAMED",
+          "--add-opens=java.base/java.util=ALL-UNNAMED",
+          "--add-opens=java.base/java.lang.invoke=ALL-UNNAMED"
+}
+
+// Add Spark 4 structured streaming source on top of the copied shared base.
+sourceSets.main.java.srcDirs += "src/main/java"
+sourceSets.test.java.srcDirs += "src/test/java"
+
+// Exclude DStream-based streaming tests from the shared base copy: the Spark 4 module
+// supports only structured streaming (batch) and does not include legacy DStream support.
+// Streaming test utilities also depend on kafka.server.KafkaServerStartable which was
+// removed in Kafka 2.8.0 (the first Kafka version with a _2.13 artifact).
+tasks.named("copyTestJava") {
+  exclude "**/translation/streaming/**"
+}
+
+// Additional supported Spark 4.x versions for compatibility tests.
+// Can be expanded as new patch releases are published.
+def sparkVersions = [
+    // "402": "4.0.2",  // primary version; already tested via the default build
+]
+
+sparkVersions.each { kv ->
+  configurations.create("sparkVersion$kv.key")
+  configurations."sparkVersion$kv.key" {
+    resolutionStrategy {
+      spark.components.each { component -> force "$component:$kv.value" }
+    }
+  }
+
+  dependencies {
+    spark.components.each { component -> "sparkVersion$kv.key" "$component:$kv.value" }
+  }
+
+  tasks.register("sparkVersion${kv.key}Test", Test) {
+    group = "Verification"
+    description = "Verifies code compatibility with Spark $kv.value"
+    classpath = configurations."sparkVersion$kv.key" + sourceSets.test.runtimeClasspath
+    systemProperties test.systemProperties
+
+    include "**/*.class"
+    maxParallelForks 4
+  }
+}
+
+tasks.register("sparkVersionsTest") {
+  group = "Verification"
+  dependsOn sparkVersions.collect{k,v -> "sparkVersion${k}Test"}
+}