Add turn item injection API (#17703)

## Summary
- Add `turn/inject_items` app-server v2 request support for appending
raw Responses API items to a loaded thread history without starting a
turn.
- Generate JSON schema and TypeScript protocol artifacts for the new
params and empty response.
- Document the new endpoint and include a request/response example.
- Preserve compatibility with the typo alias `turn/injet_items` while
returning the canonical method name.

## Testing
- Not run (not requested)

codex-rs/app-server/README.md

@@ -151,6 +151,7 @@ Example with notification opt-out:
 - `thread/backgroundTerminals/clean` — terminate all running background terminals for a thread (experimental; requires `capabilities.experimentalApi`); returns `{}` when the cleanup request is accepted.
 - `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. For `collaborationMode`, `settings.developer_instructions: null` means "use built-in instructions for the selected mode".
+- `thread/inject_items` — append raw Responses API items to a loaded thread’s model-visible history without starting a user turn; returns `{}` on success.
 - `turn/steer` — add user input to an already in-flight regular turn without starting a new turn; returns the active `turnId` that accepted the input. Review and manual compaction turns reject `turn/steer`.
 - `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"`.
 - `thread/realtime/start` — start a thread-scoped realtime session (experimental); returns `{}` and streams `thread/realtime/*` notifications. Omit `transport` for the websocket transport, or pass `{ "type": "webrtc", "sdp": "..." }` to create a WebRTC session from a browser-generated SDP offer; the remote answer SDP is emitted as `thread/realtime/sdp`.
@@ -581,6 +582,24 @@ Invoke a plugin by including a UI mention token such as `@sample` in the text in
 } } }
 ```
 
+### Example: Inject raw history items
+
+Use `thread/inject_items` to append prebuilt Responses API items to a loaded thread’s prompt history without starting a user turn. These items are persisted to the rollout and included in subsequent model requests.
+
+```json
+{ "method": "thread/inject_items", "id": 36, "params": {
+    "threadId": "thr_123",
+    "items": [
+        {
+            "type": "message",
+            "role": "assistant",
+            "content": [{ "type": "output_text", "text": "Previously computed context." }]
+        }
+    ]
+} }
+{ "id": 36, "result": {} }
+```
+
 ### Example: Start realtime with WebRTC
 
 Use `thread/realtime/start` with `transport.type: "webrtc"` when a browser or webview owns the `RTCPeerConnection` and app-server should create the server-side realtime session. The transport `sdp` must be the offer SDP produced by `RTCPeerConnection.createOffer()`, not a hand-written or minimal SDP string.