fix(codex): accept a steer only when the turn consumes it

[Tryanks]

Behaviour

Codex steers showed Steered the moment `turn/steer` was sent. That reply only confirms the app-server enqueued the input; the running turn consumes it later, before its next model request. Until then the message is still Steering… in reality.

Codex core (`session/turn.rs` → `record_pending_input` → `record_user_prompt_and_emit_turn_item`) echoes the drained input back as a `userMessage` item, so that echo is the real consumption signal. The Codex client now:

• keeps a successfully acknowledged steer as pending `(request_id, text)` instead of emitting `SteerAccepted` on the RPC reply;
• emits `SteerAccepted` when a parent-thread `userMessage` item echo's text matches a pending steer (oldest match first, so a late echo of the turn's own prompt is not mistaken for the steer);
• still refuses acceptance on an RPC error, even if matching text is later echoed.

The `SteerAccepted` contract doc in `agent/src/lib.rs` now describes the Codex signal alongside Claude's.

Tests

`steer_acceptance_waits_for_the_consumed_user_message_echo` replaces the RPC-reply test and protects: no acceptance on the `turn/steer` reply, no acceptance on a non-matching `userMessage` echo, exactly one acceptance across the started/completed echo pair, and no acceptance after an RPC error.

Checks run

• `cargo fmt --all --check` ✅
• `cargo clippy --workspace --all-targets --locked -- -D warnings` ✅
• `cargo test --workspace --locked` ✅
• Live probe against codex-cli 0.153.4: `cargo run -p agent --example probe -- codex "Run sleep 30, then reply DONE" /tmp/smoke --steer "Stop and reply PONG"` → `SteerAccepted` observed before `TurnCompleted`, marker PONG present, TurnStarted=1 TurnCompleted=1.
• Not run locally: Windows/Linux, iOS/Android/Web and `cargo machete` (CI).