* fix(server): preserve Pi turns through retries
Pi reports each provider attempt with agent_end, while agent_settled marks the final user-turn boundary. Keeping modern streams active until settlement prevents a recovered attempt from leaving the chat in an error state.
* fix(server): show Pi provider retries
Automatic retries keep the turn alive, but users still need to understand the delay. Surface Pi's retry-start signal through the existing non-terminal error activity row without changing terminal lifecycle handling.
* refactor(server): clarify Pi retry boundaries
Use the server's object-parameter convention for the retry boundary helpers so their event, turn, and retry semantics stay explicit as the lifecycle evolves.
