ci: run cargo tests on pull requests

[astrovm]

Motivation

• Ensure pull requests run the Rust test suite on GitHub Actions so repository changes are validated automatically.

Description

• Add a new workflow at .github/workflows/tests.yml that triggers on pull_request and workflow_dispatch, installs libgtk-4-dev and libadwaita-1-dev on Ubuntu, installs the stable Rust toolchain via dtolnay/rust-toolchain@stable, sets PKG_CONFIG_PATH, and runs cargo test.

Testing

• Ran an automated repository check with git diff --check which reported no issues; the new workflow itself is a CI-only change and has not yet been executed on GitHub Actions.

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Codex Task