feat(app-server): turn/steer API (#10821)

This PR adds a dedicated `turn/steer` API for appending user input to an
in-flight turn.

## Motivation
Currently, steering in the app is implemented by just calling
`turn/start` while a turn is running. This has some really weird quirks:
- Client gets back a new `turn.id`, even though streamed
events/approvals remained tied to the original active turn ID.
- All the various turn-level override params on `turn/start` do not
apply to the "steer", and would only apply to the next real turn.
- There can also be a race condition where the client thinks the turn is
active but the server has already completed it, so there might be bugs
if the client has baked in some client-specific behavior thinking it's a
steer when in fact the server kicked off a new turn. This is
particularly possible when running a client against a remote app-server.

Having a dedicated `turn/steer` API eliminates all those quirks.

`turn/steer` behavior:
- Requires an active turn on threadId. Returns a JSON-RPC error if there
is no active turn.
- If expectedTurnId is provided, it must match the active turn (more
useful when connecting to a remote app-server).
- Does not emit `turn/started`.
- Does not accept turn overrides (`cwd`, `model`, `sandbox`, etc.) or
`outputSchema` to accurately reflect that these are not applied when
steering.

codex-rs/app-server/README.md

@@ -95,6 +95,7 @@ Example (from OpenAI's official VSCode extension):
 - `thread/compact/start` — trigger conversation history compaction for a thread; returns `{}` immediately while progress streams through standard turn/item notifications.
 - `thread/rollback` — drop the last N turns from the agent’s in-memory context and persist a rollback marker in the rollout so future resumes see the pruned history; returns the updated `thread` (with `turns` populated) on success.
 - `turn/start` — add user input to a thread and begin Codex generation; responds with the initial `turn` object and streams `turn/started`, `item/*`, and `turn/completed` notifications.
+- `turn/steer` — add user input to an already in-flight turn without starting a new turn; returns the active `turnId` that accepted the input.
 - `turn/interrupt` — request cancellation of an in-flight turn by `(thread_id, turn_id)`; success is an empty `{}` response and the turn finishes with `status: "interrupted"`.
 - `review/start` — kick off Codex’s automated reviewer for a thread; responds like `turn/start` and emits `item/started`/`item/completed` notifications with `enteredReviewMode` and `exitedReviewMode` items, plus a final assistant `agentMessage` containing the review.
 - `command/exec` — run a single command under the server sandbox without starting a thread/turn (handy for utilities and validation).
@@ -363,6 +364,22 @@ You can cancel a running Turn with `turn/interrupt`.
 
 The server requests cancellations for running subprocesses, then emits a `turn/completed` event with `status: "interrupted"`. Rely on the `turn/completed` to know when Codex-side cleanup is done.
 
+### Example: Steer an active turn
+
+Use `turn/steer` to append additional user input to the currently active turn. This does not emit
+`turn/started` and does not accept turn context overrides.
+
+```json
+{ "method": "turn/steer", "id": 32, "params": {
+    "threadId": "thr_123",
+    "input": [ { "type": "text", "text": "Actually focus on failing tests first." } ],
+    "expectedTurnId": "turn_456"
+} }
+{ "id": 32, "result": { "turnId": "turn_456" } }
+```
+
+`expectedTurnId` is required. If there is no active turn (or `expectedTurnId` does not match the active turn), the request fails with an `invalid request` error.
+
 ### Example: Request a code review
 
 Use `review/start` to run Codex’s reviewer on the currently checked-out project. The request takes the thread id plus a `target` describing what should be reviewed: