[codex] add roles to realtime append text (#27936)

## Summary

Add an explicit `user` or `developer` role to
`thread/realtime/appendText` and propagate it through the realtime input
queue into `conversation.item.create`. Older JSON clients that omit the
field continue to default to `user`.

This lets app-provided context such as memory retain developer authority
without bypassing app-server through a renderer-owned data channel. The
app-server schemas, API documentation, and focused protocol and
websocket coverage are updated with the new contract.

The Codex Apps consumer is tracked in
[openai/openai#1025261](https://github.com/openai/openai/pull/1025261).

codex-rs/app-server-protocol/schema/json/codex_app_server_protocol.v2.schemas.json

@@ -4852,6 +4852,13 @@
       "title": "ContextCompactedNotification",
       "type": "object"
     },
+    "ConversationTextRole": {
+      "enum": [
+        "user",
+        "developer"
+      ],
+      "type": "string"
+    },
     "CreditsSnapshot": {
       "properties": {
         "balance": {