Retire register_bngl() + petab-spec CI machinery once a PyPI petab after 0.8.2 ships native BNGL

[wshlavacek]

Native BNGL support (BnglModel) merged into upstream libpetab-python in PEtab-dev/libpetab-python#508 and now lives on main. However, the latest PyPI release (petab 0.8.2, 2026-03-12) does not carry it — #508 merged after that tag. So PyBNF still ships its local fallback.

Trigger: the first PyPI petab release after 0.8.2 whose known_model_types includes bngl.

When that lands, do the teardown (verify no other consumers before each deletion):

• Bump the petab pin in pyproject.toml ([petab] and [tests] extras) to >=<that release>.
• Remove the register_bngl() monkeypatch and the local Step A adapter (pybnf/petab/bngl_model.py) — confirm pybnf/petab/_bngl.py's parse_model has no remaining runtime consumers before removing the reader.
• Drop the dedicated pytest-native-bngl CI leg and the petab-spec override in .github/actions/setup-pybnf/action.yml — with stock petab now native, the default matrix legs exercise the native loader directly.
• Update the register_bngl()-referencing tests to drive the native loader directly.
• Refresh the drift note in pybnf/petab/_bngl.py and the pyproject.toml comments.

Context: lanl/PyBNF#590 repointed the native-BNGL CI leg from the (now-superseded) fork branch to upstream main; that keeps us green against the merge but leaves the shipped fallback in place until this issue's trigger fires.