ci: enable tests and coverage in GitHub Actions

Add test execution and coverage reporting to quality-checks job:
- Run tests via `just test cpy314`
- Run coverage report via `just check-coverage cpy314`

Note: This work was completed with AI assistance (Claude Code).

Author: oberstet

.github/workflows/main.yml

@@ -66,8 +66,11 @@ jobs:
       - name: Run static type checking with ty
         run: just check-typing cpy314
 
-      # - name: Run tests and generate an HTML coverage report
-      #   run: just check-coverage cpy314
+      - name: Run tests
+        run: just test cpy314
+
+      - name: Run tests with coverage report
+        run: just check-coverage cpy314
 
   documentation:
     name: Documentation Build