ci: run Maven tests on pull requests

[omer-cengel]

Summary

Add a dedicated pull-request workflow so changes targeting master run the complete Maven test lifecycle before merge.

Changes

• Trigger the Tests workflow for pull requests targeting master.
• Run on Ubuntu with Temurin Java 21 and Maven dependency caching.
• Execute mvn --batch-mode --no-transfer-progress test with a ten-minute timeout.
• Use read-only repository permissions and disable persisted checkout credentials.
• Cancel superseded Tests runs for the same pull request with concurrency isolated from formatting checks.

Scope and non-goals

• Adds only the dedicated test workflow and leaves the existing formatting workflow unchanged.
• Does not add push or manual triggers, platform or Java matrices, coverage, artifacts, deployment, or branch-protection configuration.