ci: skip enforce-banned-duplicate-classes in gax-bom and first-party-dependencies

In #14142 and #14272, the enforcer was split into a fast PR check (enforce)
and a release check (enforce-banned-duplicate-classes). Modules without this
execution ID or rules fail with "No rules are configured".

gax-bom and first-party-dependencies are POM-only packaging modules that
inherit a released version of google-cloud-shared-config (1.17.0) from
Maven Central to avoid monorepo publication ordering cycles. Because
1.17.0 does not define the enforce-banned-duplicate-classes execution,
the ban-duplicate-classes release CI job failed on these two modules.

Add an explicit skip execution for enforce-banned-duplicate-classes to both
POMs, matching the existing pattern in native-image-shared-config and
google-cloud-shared-config-parent.

Author: blakeli0

sdk-platform-java/gax-java/gax-bom/pom.xml

@@ -120,6 +120,21 @@
           </execution>
         </executions>
       </plugin>
+      <!-- This POM-only module inherits a released version of google-cloud-shared-config from Central
+           which does not define the enforce-banned-duplicate-classes execution. Skip it here to prevent
+           "No rules are configured" errors when invoking enforcer:enforce@enforce-banned-duplicate-classes. -->
+      <plugin>
+        <groupId>org.apache.maven.plugins</groupId>
+        <artifactId>maven-enforcer-plugin</artifactId>
+        <executions>
+          <execution>
+            <id>enforce-banned-duplicate-classes</id>
+            <configuration>
+              <skip>true</skip>
+            </configuration>
+          </execution>
+        </executions>
+      </plugin>
     </plugins>
   </build>
 </project>

sdk-platform-java/java-shared-dependencies/first-party-dependencies/pom.xml

@@ -79,4 +79,24 @@
       </dependency>
     </dependencies>
   </dependencyManagement>
+
+  <build>
+    <plugins>
+      <!-- This POM-only module inherits a released version of google-cloud-shared-config from Central
+           which does not define the enforce-banned-duplicate-classes execution. Skip it here to prevent
+           "No rules are configured" errors when invoking enforcer:enforce@enforce-banned-duplicate-classes. -->
+      <plugin>
+        <groupId>org.apache.maven.plugins</groupId>
+        <artifactId>maven-enforcer-plugin</artifactId>
+        <executions>
+          <execution>
+            <id>enforce-banned-duplicate-classes</id>
+            <configuration>
+              <skip>true</skip>
+            </configuration>
+          </execution>
+        </executions>
+      </plugin>
+    </plugins>
+  </build>
 </project>