Build: avoid empty directories in JARs

[ajantha-bhat]

Summary

Avoid adding empty gradle/ and gradle/jar-licenses/ directories to JARs when copying LICENSE and NOTICE into META-INF.

Problem

The existing copy configuration starts at the repository root. Gradle retains the source directories as empty JAR entries:

META-INF/LICENSE
META-INF/NOTICE
gradle/
gradle/jar-licenses/

Fix

Copy the legal files directly from gradle/jar-licenses into META-INF for shared Java JARs and the Spark 3.5 and 4.0 JARs. Spark bundle JAR handling is unchanged.

Verification

• Confirmed main, sources, and Javadoc JARs contain exactly one META-INF/LICENSE and META-INF/NOTICE.
• Confirmed gradle/ and gradle/jar-licenses/ are absent after the fix.
• Confirmed custom module LICENSE handling is preserved.
• Confirmed Spark bundle JARs still contain their customized legal files.
• ./gradlew spotlessCheck passes.

Checklist

• 🛡️ Don't disclose security issues! (contact security@apache.org)
• 🔗 Clearly explained why the changes are needed, or linked related issues: Fixes #
• 🧪 Added/updated tests with good coverage, or manually tested (and explained how)
• 💡 Added comments for complex logic
• 🧾 Updated CHANGELOG.md (if needed)
• 📚 Updated documentation in site/content/in-dev/unreleased (if needed)

[ajantha-bhat]

Found during polaris tools release

apache/polaris-tools#270

[ajantha-bhat]

I will take a look at flaky test later.