[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/codex-api/src/endpoint/realtime_websocket/methods_common.rs

@@ -13,6 +13,7 @@ use crate::endpoint::realtime_websocket::protocol::RealtimeSessionConfig;
 use crate::endpoint::realtime_websocket::protocol::RealtimeSessionMode;
 use crate::endpoint::realtime_websocket::protocol::RealtimeVoice;
 use crate::endpoint::realtime_websocket::protocol::SessionUpdateSession;
+use codex_protocol::protocol::ConversationTextRole;
 use serde_json::Result as JsonResult;
 use serde_json::Value;
 use serde_json::to_value;
@@ -33,10 +34,11 @@ pub(super) fn normalized_session_mode(
 pub(super) fn conversation_item_create_message(
     event_parser: RealtimeEventParser,
     text: String,
+    role: ConversationTextRole,
 ) -> RealtimeOutboundMessage {
     match event_parser {
-        RealtimeEventParser::V1 => v1_conversation_item_create_message(text),
-        RealtimeEventParser::RealtimeV2 => v2_conversation_item_create_message(text),
+        RealtimeEventParser::V1 => v1_conversation_item_create_message(text, role),
+        RealtimeEventParser::RealtimeV2 => v2_conversation_item_create_message(text, role),
     }
 }