fix: align Spring smoke test protobuf runtime

Signed-off-by: Gregor Zeitlinger <gregor.zeitlinger@grafana.com>

Author: zeitlinger

integration-tests/it-spring-boot-smoke-test/pom.xml

@@ -21,10 +21,17 @@
   <properties>
     <java.version>25</java.version>
     <junit-jupiter.version>6.1.3</junit-jupiter.version>
+    <!-- Keep aligned with generated sources when reactor dependencies use target/classes. -->
+    <protobuf-java.version>4.35.1</protobuf-java.version>
   </properties>
 
   <dependencyManagement>
     <dependencies>
+      <dependency>
+        <groupId>com.google.protobuf</groupId>
+        <artifactId>protobuf-java</artifactId>
+        <version>${protobuf-java.version}</version>
+      </dependency>
       <dependency>
         <groupId>org.junit</groupId>
         <artifactId>junit-bom</artifactId>