chore: replace deprecated codecov with GitHub Action (#2747)

Migrate from the deprecated codecov npm package and bash uploader to the
official codecov/codecov-action@v5. This fixes gcov errors on macOS runners
and provides better reliability for coverage uploads.

Co-authored-by: Claude Opus 4.5 <noreply@anthropic.com>

.github/workflows/push_tests.yml

@@ -69,10 +69,21 @@ jobs:
       - name: Install dependencies
         run: make pydist install-dist dev-env-server
       - name: Unit tests
-        run: |
-          make unit-test-server unit-test-client
-          bash <(curl -s https://codecov.io/bash) -y .codecov.yml -k server -cF server,python,unitTest
-          cd client && ./node_modules/codecov/bin/codecov --yml=../.codecov.yml --root=../ --gcov-root=../ -C -F frontend,javascript,unitTest
+        run: make unit-test-server unit-test-client
+      - name: Generate server coverage XML
+        run: coverage xml -o server/coverage.xml
+      - name: Upload server coverage
+        uses: codecov/codecov-action@v5
+        with:
+          flags: server,python,unitTest
+          files: ./server/coverage.xml
+          fail_ci_if_error: false
+      - name: Upload client coverage
+        uses: codecov/codecov-action@v5
+        with:
+          flags: frontend,javascript,unitTest
+          files: ./client/coverage/lcov.info
+          fail_ci_if_error: false
 
   smoke-tests:
     runs-on: macos-latest
@@ -102,9 +113,7 @@ jobs:
           pip install setuptools
           make pydist install-dist
       - name: Smoke tests (without annotations feature)
-        run: |
-          cd client && make smoke-test
-          ./node_modules/codecov/bin/codecov --yml=../.codecov.yml --root=../ --gcov-root=../ -C -F frontend,javascript,smokeTest
+        run: cd client && make smoke-test
 
   # TODO: reinstate: https://github.com/chanzuckerberg/cellxgene/issues/2544
   # smoke-tests-annotations: