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-protocol/schema/json/ClientRequest.json

@@ -2810,6 +2810,29 @@
       ],
       "type": "object"
     },
+    "TurnSteerParams": {
+      "properties": {
+        "expectedTurnId": {
+          "description": "Required active turn id precondition. The request fails when it does not match the currently active turn.",
+          "type": "string"
+        },
+        "input": {
+          "items": {
+            "$ref": "#/definitions/UserInput"
+          },
+          "type": "array"
+        },
+        "threadId": {
+          "type": "string"
+        }
+      },
+      "required": [
+        "expectedTurnId",
+        "input",
+        "threadId"
+      ],
+      "type": "object"
+    },
     "UserInput": {
       "oneOf": [
         {
@@ -3511,6 +3534,30 @@
       "title": "Turn/startRequest",
       "type": "object"
     },
+    {
+      "properties": {
+        "id": {
+          "$ref": "#/definitions/RequestId"
+        },
+        "method": {
+          "enum": [
+            "turn/steer"
+          ],
+          "title": "Turn/steerRequestMethod",
+          "type": "string"
+        },
+        "params": {
+          "$ref": "#/definitions/TurnSteerParams"
+        }
+      },
+      "required": [
+        "id",
+        "method",
+        "params"
+      ],
+      "title": "Turn/steerRequest",
+      "type": "object"
+    },
     {
       "properties": {
         "id": {