Lock in that recording a producer never moves the pinned model
No code change: nothing writes conv.model from routerMetadata, and the
GET handler returns conversation.model verbatim while findCurrentModel
resolves the picker from that id alone. Under the omni alias the pin
therefore stays omni across a revisit even though each turn records the
candidate that actually answered.
Adds a regression test, since the producer backfill is new and it would
be easy to later 'helpfully' derive the pin from the last turn.
