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-protocol/schema/json/v2/ThreadInjectItemsParams.json

@@ -0,0 +1,19 @@
+{
+  "$schema": "http://json-schema.org/draft-07/schema#",
+  "properties": {
+    "items": {
+      "description": "Raw Responses API items to append to the thread's model-visible history.",
+      "items": true,
+      "type": "array"
+    },
+    "threadId": {
+      "type": "string"
+    }
+  },
+  "required": [
+    "items",
+    "threadId"
+  ],
+  "title": "ThreadInjectItemsParams",
+  "type": "object"
+}