fix(api)!: correct model types and network policy docs (#3896)

## Summary

Removes the unsupported gpt-5.1-mini model from typed model choices and
clarifies network policy and Realtime documentation. The model was never
available; removing its typed choices is an intentional breaking change.

## Changes

- Remove gpt-5.1-mini from typed model choices
- Document version-specific network defaults and the difference between
omitting a policy and resetting it with null
- Identify conversation.item.added and conversation.item.done in
Realtime item-creation documentation
- Clarify recovery and completion semantics for agent session
cancellation and deletion

Co-authored-by: markstuart-oai <323302876+markstuart-oai@users.noreply.github.com>