Implement Test Coverage Measurement and Reporting

.changesets/maint_coverage.md

@@ -0,0 +1,3 @@
+### Implement Test Coverage Measurement and Reporting - @DaleSeo PR #335
+
+This PR adds the bare minimum for code coverage reporting using [cargo-llvm-cov](https://crates.io/crates/cargo-llvm-cov) and integrates with [Codecov](https://www.codecov.io/). It adds a new `coverage` job to the CI workflow that generates and uploads coverage reporting in parallel with existing tests. The setup mirrors that of Router, except it uses `nextest` instead of the built-in test runner and CircleCI instead of GitHub Actions.

.github/workflows/ci.yml

@@ -124,3 +124,22 @@ jobs:
           gc-max-store-size: 5G
       - name: Run Tests
         run: 'nix develop --command bash -c "cargo test"'
+
+  coverage:
+    name: Run Coverage
+    runs-on: ubuntu-24.04
+    permissions:
+      contents: read
+    steps:
+      - uses: actions/checkout@v5
+        with:
+          ref: ${{ github.event.pull_request.head.sha }}
+      - uses: taiki-e/install-action@cargo-llvm-cov
+      - name: Generate code coverage
+        run: cargo llvm-cov --all-features --workspace --codecov --output-path codecov.json
+      - name: Upload coverage to Codecov
+        uses: codecov/codecov-action@v5
+        with:
+          token: ${{ secrets.CODECOV_TOKEN }} # not required for public repos
+          files: codecov.json
+          fail_ci_if_error: true