When runWithModelFallback falls back to a secondary provider it writes
providerOverride/modelOverride/modelOverrideSource:"auto" to the session.
On subsequent turns createModelSelectionState read this stored override and
passed the fallback provider directly to runWithModelFallback, so the
configured primary was never retried — the session was permanently pinned to
the fallback even after the primary recovered.
Fix: at model-selection ingress, when the direct session override has
modelOverrideSource "auto" (set by a previous automatic fallback, not a user
/model command), clear the override and retry the configured primary. If the
primary is still down runWithModelFallback will fall back and re-set the auto
override for that turn. Once the primary recovers the override stays clear.
User-selected overrides (modelOverrideSource "user" or legacy undefined+model)
are preserved unchanged.
Covered by four new unit tests in model-selection.test.ts:
- auto-failover override cleared and primary retried
- user-selected override preserved
- legacy override without source field preserved
- parent-session auto-override applied to child (not cleared by child logic)
f2abe28d40