ci: upload coverage to Codecov

Emit coverage.xml from pytest (--cov is already configured in
pyproject.toml) and upload it on the Python floor matrix entry only.
Non-blocking so a Codecov outage cannot break CI.

Requires CODECOV_TOKEN to be set as a repo secret.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: nficano

.github/workflows/test.yml

@@ -74,7 +74,20 @@ jobs:
         run: uv run pyright
 
       - name: Pytest
-        run: uv run pytest --junitxml=junit-${{ matrix.python-version }}.xml
+        run: uv run pytest --junitxml=junit-${{ matrix.python-version }}.xml --cov-report=xml
+
+      # Upload coverage from the floor matrix entry only — the report is the
+      # same on every Python version, so one upload is enough. Non-blocking:
+      # a Codecov outage must not break CI.
+      - name: Upload coverage to Codecov
+        if: matrix.python-version == '3.13'
+        # codecov/codecov-action v6.0.1
+        uses: codecov/codecov-action@e79a6962e0d4c0c17b229090214935d2e33f8354 # v6.0.1
+        with:
+          fail_ci_if_error: false
+          flags: unittests
+          files: ./coverage.xml
+          token: ${{ secrets.CODECOV_TOKEN }}
 
       - name: Upload junit XML on failure
         if: failure()