[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/src/request_processors/turn_processor.rs

@@ -997,7 +997,10 @@ impl TurnRequestProcessor {
         self.submit_core_op(
             request_id,
             thread.as_ref(),
-            Op::RealtimeConversationText(ConversationTextParams { text: params.text }),
+            Op::RealtimeConversationText(ConversationTextParams {
+                text: params.text,
+                role: params.role,
+            }),
         )
         .await
         .map_err(|err| {