Fix copyright header placement in IndexMetadata.java

[Copilot]

What does this PR do?

Moves the copyright header to the beginning of IndexMetadata.java, before the package statement, following standard Java conventions. Also adds SPDX license identifiers to match repository formatting.

Motivation

Addresses review feedback from #2828 - the copyright header was incorrectly placed after the package and import statements.

Related issues

• Parent PR: #2828
• Review comment: https://github.com/ArcadeData/arcadedb/pull/2828#discussion_r2560654146

Additional Notes

None.

Checklist

• [ ] I have run the build using mvn clean package command
• [ ] My unit tests cover both failure and success scenarios

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  • Triggering command: /usr/lib/jvm/temurin-17-jdk-amd64/bin/java /usr/lib/jvm/temurin-17-jdk-amd64/bin/java -classpath /home/REDACTED/.m2/wrapper/dists/apache-maven-3.9.9/3477a4f1/boot/plexus-classworlds-2.8.0.jar -Dclassworlds.conf=/home/REDACTED/.m2/wrapper/dists/apache-maven-3.9.9/3477a4f1/bin/m2.conf -Dmaven.home=/home/REDACTED/.m2/wrapper/dists/apache-maven-3.9.9/3477a4f1 -Dlibrary.jansi.path=/home/REDACTED/.m2/wrapper/dists/apache-maven-3.9.9/3477a4f1/lib/jansi-native -Dmaven.multiModuleProjectDirectory=/home/REDACTED/work/arcadedb/arcadedb org.codehaus.plexus.classworlds.launcher.Launcher -f pom.xml -B -V -e -Dfindbugs.skip -Dcheckstyle.skip -Dpmd.skip=true -Dspotbugs.skip -Denforcer.skip -Dmaven.javadoc.skip -DskipTests (dns block)

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