add placeholder for system test nox session

Author: ohmayr

packages/gapic-generator/noxfile.py

@@ -808,6 +808,15 @@ def format(session):
         *LINT_PATHS,
     )
 
+
+@nox.session(python=ALL_PYTHON)
+def system(session):
+    # TODO(https://github.com/googleapis/google-cloud-python/issues/16190):
+    # Implement system test session.
+    """Run the system test suite (skipped for migration)."""
+    session.skip(f"system session is not yet implemented for gapic-generator-python.")
+
+
 @nox.session(python=NEWEST_PYTHON)
 @nox.parametrize(
     "protobuf_implementation",
@@ -831,4 +840,4 @@ def core_deps_from_source(session, protobuf_implementation):
     """Run all tests with core dependencies installed from source."""
     # TODO(https://github.com/googleapis/google-cloud-python/issues/16185): 
     # Implement logic to install core packages directly from the mono-repo directories.
-    session.skip("core_deps_from_source session is not yet implemented for gapic-generator-python.")
+    session.skip("core_deps_from_source session is not yet implemented for gapic-generator-python."