Previously /v1/api/models (home composer placeholder) and
console/session_factory.py walked separate two-/three-tier chains for
the coordinator alias. session_factory was missing the
``model.default_alias`` tier, so admins who set the system default in
the Models tab would see it advertised but new coordinator sessions
would silently keep launching on ``registry.default``.
This commit:
- Extracts the chain into ``turnstone/console/coordinator_alias.py``.
``resolve_coordinator_alias`` returns the effective alias under a
shared three-tier policy: explicit pin → ``model.default_alias`` →
``registry.default``. Tier 2 is validated against
``registry.has_alias`` and falls through to tier 3 with a logged
warning if unknown. Tier 1 is intentionally passed through
unvalidated so an explicit operator pin surfaces as 503 at
``registry.resolve`` rather than being silently swapped out.
- Wires both call sites through the helper. The placeholder supplies
an ``alias_filter`` that restricts every tier to enabled DB rows so
the home composer never advertises a model the workstream picker
can't actually offer; the session factory uses no filter (matches
prior 503-on-typo behaviour for explicit pins).
- Adds direct integration tests for the session factory's chain
(``tests/test_console_session_factory.py``) and updates the
placeholder tests' fixture to provide a stub coord_registry, since
the helper now requires one.
Patrick Buckley