The test was restoring original_model from /api/models, but after prior runs
the config.yaml model.default field could be stale, causing the restore to
bake in the wrong value. Fix: always restore to TEST_DEFAULT_MODEL (the
conftest-injected env value) for deterministic ordering-independent cleanup.
Also exposes TEST_DEFAULT_MODEL from _pytest_port.py so other tests that
mutate the default model can use it for clean teardown.
TESTING.md: update automated test count from 1353 to 1578.
