ci: inline protobuf-maven-plugin version in itk/pom.xml

Author: JakubWorek

itk/pom.xml

@@ -106,7 +106,6 @@
           -Da2a.itk.proto.dir=... when building outside run_itk.sh.
         -->
         <a2a.itk.proto.dir>${project.basedir}/a2a-itk/protos</a2a.itk.proto.dir>
-        <protobuf-maven-plugin.version>4.1.2</protobuf-maven-plugin.version>
     </properties>
 
     <build>
@@ -118,10 +117,15 @@
                   by a2a-itk/agents/java/v10 so generated code (package `itk`,
                   outer class `InstructionOuterClass`) stays byte-identical
                   across the two paths during the strangler migration.
+                  Version is pinned inline to match spec-grpc/pom.xml (same
+                  plugin, same version) rather than reusing the parent's
+                  protobuf-maven-plugin.version property, which resolves to
+                  the org.xolstice plugin's version scheme (0.6.x) used by
+                  compat-0.3/spec-grpc.
                 -->
                 <groupId>io.github.ascopes</groupId>
                 <artifactId>protobuf-maven-plugin</artifactId>
-                <version>${protobuf-maven-plugin.version}</version>
+                <version>4.1.2</version>
                 <configuration>
                     <protoc>${protobuf-java.version}</protoc>
                     <sourceDirectories>