[#420] Add Java 25 support

[swlodarski-sumoheavy]

Summary

This PR adds Java 25 support to the build matrix and updates GitHub Actions to their latest versions. Additionally, includes a bug fix for handling java.io.FileNotFoundException in KeyUtils.

Changes

1. Java 25 Support
   • Added Java 25 to build matrix in build.yml
   • Ensures the SDK is tested and compatible with the latest Java version
2. GitHub Actions Updates
   • Updated actions/checkout: v4 → v6
   • Updated actions/setup-java: v4 → v5
   • Ensures CI/CD pipelines use the latest action versions with improved performance and security
3. Bug Fix in KeyUtils
   • Enhanced null safety in privateKeyExists() method
   • Added null/empty check for privateKeyFile parameter before file existence check
   • Prevents potential NullPointerException when null or empty string is passed
   • Returns false early if the parameter is invalid

Testing

• All tests pass on Java 8, 11, 17, 21, 23, and 25
• Build succeeds on all supported Java versions
• Checkstyle validation passes

Compatibility

• Maintains backward compatibility with Java 8
• No breaking changes to public APIs