ci: separate lint and test jobs, trigger only on PR opened

[soraiayugulis]

Summary

Optimizes the PR validation workflow by separating lint and test into independent parallel jobs and restricting the trigger to PR open events only.

Changes

Workflow (pr-validation.yml)

• Trigger: Changed from pull_request (all types) to pull_request: types: [opened] — runs only when a PR is first opened
• Split into two parallel jobs:
  • lint — runs mvn checkstyle:check
  • test — runs mvn test
• Both jobs run independently and in parallel, reducing overall CI time

Build (pom.xml)

• Added maven-checkstyle-plugin 3.3.1 with google_checks.xml
• Configured to run during validate phase
• Fails build on checkstyle violations

Rationale

• Faster feedback: lint and test run in parallel instead of sequentially
• Clearer failures: lint issues and test failures are reported independently
• Reduced noise: CI only triggers on PR open, not on every push to the branch