SessionProcessor retry duplicates

Trap: retrying the complete llm.stream(...) after a retryable timeout, network failure, rate limit, or 5xx looks safe because no terminal response arrived. The provider may already have accepted the request or executed a tool before the failure was observed. Fix: treat started non-terminal attempts and state-changing tools as uncertain unless protected by provider idempotency or a durable invocation ledger; never infer replay safety from missing projected output.