[codex] Remove external client session reset plumbing

[pakrym-oai]

Why

The turn loop no longer needs to decide when a ModelClientSession should reset its websocket state after compaction. That reset behavior belongs inside the model client, where the websocket cache and retry state are owned. The repo guidance now calls this out explicitly so future changes let the incremental request logic decide whether the previous request can be reused.

What Changed

• Removed the reset_client_session return value from pre-sampling and auto-compact helpers in core/src/session/turn.rs.
• Changed compaction helpers to return CodexResult<()> so callers only handle success or failure.
• Made ModelClientSession::reset_websocket_session private to core/src/client.rs, leaving it callable only from model-client internals.
• Added AGENTS.md guidance not to call reset_client_session unnecessarily.

Validation

• just test -p codex-core session::turn