Consider run/turn-aware session history retrieval instead of item-only limit

[houtaroy]

this issue is similar to #1385

A single user turn may produce multiple persisted items:

1. user input
2. function calling
3. function calling output
4. assistant output

conversation strategy session, with item-based limit, the retrieved history can begin in the middle of that sequence, for example including a function calling output without its corresponding function calling, or an assistant output without the user request that triggered it.

can we do:

1. Persist a run_id / turn_id with each session item
2. Add run_limit / turn_limit for get_items, returning complete runs messages