[Infra] Add CI pipeline for automated testing on PRs

[ABresting]

Context

There is currently no CI pipeline that runs tests automatically when a PR is opened or updated. Reviewers have to pull the branch and run tests locally to verify nothing is broken. An automated test workflow would catch regressions early and speed up the review cycle.

Scope

• GitHub Actions workflow that triggers on PR open/push to key branches
• Run cargo test --workspace and report pass/fail status on the PR
• Run cargo clippy for lint checks
• Cache Cargo dependencies and build artifacts for faster runs
• Block merge on test failure (branch protection)