Upstream fixture surface, wave 2: testing_team and mock_supervisor_comms

[nredd]

#227 round 3: the six names from #237 are all shipped and their error classes are at ZERO across the matrix. What remains (s2 unit/models setup errors):

Cases:

• testing_team not found -- 4 (3.1.0), 4 (3.1.8), 21 (3.2.2), 41 (3.3.1), 41 (3.4.0.dev). The multi-team fixture upstream added in the 3.3 era; the count doubles at 3.3+ and this is now the largest missing-fixture class. E.g. test_trigger.py::test_trigger_team_name_stored, test_variable.py::TestVariable::test_set_variable_sets_team
• mock_supervisor_comms not found -- 3 flat on 3.2+. E.g. test_xcom.py::TestXCom::test_set_serialize_call_current_signature
• reset_logging_config / clean_executor_loader -- 1-2 each, tests_common-internal plumbing; IMO out of scope, listing for completeness

Logs here. CC #227, successor to #237

[nredd]

Shelving this for now -- not implementing. It's almost purely upstream-parity: testing_team has no end-user story (multi-team is Airflow-internal, counts come entirely from the #227 shim runs) and the surface is still moving upstream (3.4.0.dev already changed the fixture's teardown, so parity now means chasing drift later). mock_supervisor_comms is marginal too -- run_task/task_context already cover the comms path for real consumers.

Archiving the plan sketch for whenever this thaws:

• testing_team: fixtures/upstream.py next to testing_dag_bundle, new _compat/team.py ensure_shared_team modeled on ensure_shared_bundle (get-or-create, IntegrityError race tolerated, no teardown delete). Schema break is exactly 3.2.0 (3.1.x: UUID id pk, no default; 3.2.0+: name pk) -- new certified capability team_model_identity, structural probe 'id' in Team.__table__.columns
• mock_supervisor_comms: autospec CommsDecoder monkeypatched onto task_runner.SUPERVISOR_COMMS via _compat/in_process.py; the Task SDK <1.2 fallback is dead code on our floor. Document the airflow_variables/airflow_connections seeding blackhole
• reset_logging_config/clean_executor_loader: out of scope regardless, tests_common harness internals

Revisit if a future #227 round shows the counts growing or a real consumer asks.