Three parity fixes between the API server and the native gateway's
agent-runtime resolution, integrated with the provider-aware request
routing that landed in #70853:
- Session-persisted model is honored: POST /api/sessions {"model": ...}
stores a model that the chat handlers previously fetched and threw
away. A stored value that matches a model_routes alias goes through
the route path (route provider/credentials apply); a raw model string
threads through as session_model, pinning the session's turns ahead
of per-request body values but below an explicit session /model
override.
- Empty-model recovery: provider-catalog default when config has no
model.default but a provider resolved, plus last-known-good model
recovery (#35314) keyed on gateway_session_key only (never ephemeral
session_id — no unbounded growth from one-off requests).
- Provider auth failures surface as controlled responses: RuntimeError
from _resolve_runtime_agent_kwargs() is re-raised as a dedicated
_ProviderAuthResolutionError at the call site, caught narrowly in
_run_agent() and the /v1/runs executor to return run.py's response
shape instead of an undifferentiated 500 (session-chat endpoints
previously returned a raw aiohttp 500 with no JSON body).
Salvaged from PR #57947 by @FvanW; session-model route-alias resolution
from PR #59941 by @kaishi00.
Co-authored-by: kaishi00 <kaishi00@users.noreply.github.com>
