[codex] Remove external websocket session resets

[pakrym-oai]

Why

Compaction now installs replacement history inside the session, but the turn and compaction callers were still reaching into ModelClientSession to reset websocket transport state after that install. That made a transport-level reset part of the compaction API even though websocket incremental request selection already checks whether the next request is a strict extension of the previous one and falls back to a full response.create when it is not.

What changed

• Removed the compaction-side calls to reset_websocket_session from compact.rs and session/turn.rs.
• Simplified pre-sampling and mid-turn compaction helpers so they return CodexResult<()> instead of carrying a reset flag.
• Made ModelClientSession::reset_websocket_session private to client.rs, leaving only the websocket timeout recovery path inside the client as a caller.

Validation

• cargo test -p codex-core --test all responses_websocket_creates_on_non_prefix
• cargo test -p codex-core --test all steered_user_input_waits_for_model_continuation_after_mid_turn_compact
• cargo test -p codex-core --test all pre_sampling_compact_runs_on_switch_to_smaller_context_model