Add test coverage measurement and CodeCov upload to CI

- Add --cov-report=xml to tox.ini so pytest-cov emits coverage.xml
- Add codecov/codecov-action@v5 step to gating.yaml to upload coverage
  results to CodeCov using OIDC authentication

Closes #76
Generated-By: OpenCode (google-vertex-anthropic/claude-sonnet-4-6@default)

Author: Agent

.github/workflows/gating.yaml

@@ -35,3 +35,9 @@ jobs:
         run: tox -e docs
       - name: tests
         run: tox -e py3
+      - uses: codecov/codecov-action@v5
+        with:
+          use_oidc: true
+          flags: unit-tests
+          files: ./coverage.xml
+          fail_ci_if_error: false

tox.ini

@@ -19,6 +19,7 @@ commands =
         -W "ignore:inspect.getargspec:DeprecationWarning" \
         -W "ignore:This method will be removed in future versions.  Use 'parser.read_file()':DeprecationWarning" \
         -W "ignore:Use .persist_selectable:DeprecationWarning" \
+        --cov-report=xml \
         {posargs}
 sitepackages = True