Have nox run the legacy protobuf test

Author: vchudnov-g

.github/workflows/unittest.yml

@@ -45,6 +45,9 @@ jobs:
           - "3.12"
           - "3.13"
           - "3.14"
+        option:
+          - ""
+          - "_protobuf_4x"
     steps:
     - name: Checkout
       uses: actions/checkout@v4
@@ -59,14 +62,14 @@ jobs:
         python -m pip install nox
     - name: Run unit tests
       env:
-        COVERAGE_FILE: .coverage-${{matrix.python }}
+        COVERAGE_FILE: .coverage${{ matrix.option }}-${{matrix.python }}
       run: |
-        nox -s unit-${{ matrix.python }}
+        nox -s unit${{ matrix.option }}-${{ matrix.python }}
     - name: Upload coverage results
       uses: actions/upload-artifact@v4
       with:
         name: coverage-artifact-${{ matrix.python }}
-        path: .coverage-${{ matrix.python }}
+        path: .coverage${{ matrix.option }}-${{ matrix.python }}
         include-hidden-files: true
 
   report-coverage:

noxfile.py

@@ -252,9 +252,12 @@ def unit(session, install_grpc_gcp, install_grpc, install_async_rest):
 
 
 # TODO: Remove once we stop support for protobuf 4.x.
-@nox.session(python="3.11")
+@nox.session(python=PYTHON_VERSIONS)
 def unit_protobuf_4x(session):
     """Run the unit test suite with protobuf 4.x."""
+    if session.python > "3.11":
+        session.log("Skipping session: protobuf 4.x only runs on Python < 3.12")
+        session.skip()
     # Pin protobuf to a 4.x version to ensure coverage for the legacy code path.
     session.install("protobuf>=4.25.8,<5.0.0")
     default(session, install_grpc=True)