Add flatten-maven-plugin to maven-central profile for Sonatype POM validation

Child modules inherit metadata (url, licenses, scm, developers) from the parent
POM, but Maven Central validates the uploaded POM file directly. The flatten-maven-plugin
with ossrh mode creates a flattened POM that includes all inherited metadata and
resolved dependency versions, satisfying Sonatype Central Portal validation requirements.

Author: ancongui

pom.xml

@@ -110,6 +110,7 @@
         <maven-enforcer-plugin.version>3.6.2</maven-enforcer-plugin.version>
         <maven-gpg-plugin.version>3.2.8</maven-gpg-plugin.version>
         <central-publishing-maven-plugin.version>0.10.0</central-publishing-maven-plugin.version>
+        <flatten-maven-plugin.version>1.6.0</flatten-maven-plugin.version>
     </properties>
 
     <dependencyManagement>
@@ -466,6 +467,30 @@
                             <checksums>required</checksums>
                         </configuration>
                     </plugin>
+                    <plugin>
+                        <groupId>org.codehaus.mojo</groupId>
+                        <artifactId>flatten-maven-plugin</artifactId>
+                        <version>${flatten-maven-plugin.version}</version>
+                        <configuration>
+                            <flattenMode>ossrh</flattenMode>
+                        </configuration>
+                        <executions>
+                            <execution>
+                                <id>flatten</id>
+                                <phase>process-resources</phase>
+                                <goals>
+                                    <goal>flatten</goal>
+                                </goals>
+                            </execution>
+                            <execution>
+                                <id>flatten.clean</id>
+                                <phase>clean</phase>
+                                <goals>
+                                    <goal>clean</goal>
+                                </goals>
+                            </execution>
+                        </executions>
+                    </plugin>
                 </plugins>
             </build>
         </profile>