[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/src/protocol/v2/realtime.rs

@@ -1,3 +1,4 @@
+use codex_protocol::protocol::ConversationTextRole;
 use codex_protocol::protocol::RealtimeAudioFrame as CoreRealtimeAudioFrame;
 use codex_protocol::protocol::RealtimeConversationArchitecture;
 use codex_protocol::protocol::RealtimeConversationVersion;
@@ -135,6 +136,8 @@ pub struct ThreadRealtimeAppendAudioResponse {}
 pub struct ThreadRealtimeAppendTextParams {
     pub thread_id: String,
     pub text: String,
+    #[serde(default)]
+    pub role: ConversationTextRole,
 }
 
 /// EXPERIMENTAL - response for appending realtime text input.