Route /fast eligibility through _resolve_session_agent_runtime (the same
resolver normal turns use) instead of the global _resolve_gateway_model,
so a session-scoped /model override or channel override keeps /fast in
sync with the model the next agent turn actually routes to. Normalize the
command source exactly like /model does so the override is read under the
key it was stored, and let the resolver rehydrate persisted overrides a
gateway restart cleared from memory.
Rebased onto current main (was DIRTY): the lazy import in
_handle_fast_command now pulls only _load_gateway_config; the sibling
picker tests in test_choice_picker.py::TestFastChoicePicker are updated to
stub _resolve_runtime_agent_kwargs (matching test_fast_command.py) so the
no-override path doesn't hit real provider resolution in CI.
Tests: tests/gateway/test_fast_command.py (10) +
tests/gateway/test_choice_picker.py::TestFastChoicePicker (4) pass.
(cherry picked from commit 9885267a6f6e124b470fa2f597ef9c3623063549)
cd2c5cf9af