[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/protocol/src/protocol.rs

@@ -401,6 +401,16 @@ pub struct ConversationAudioParams {
 #[derive(Debug, Clone, PartialEq)]
 pub struct ConversationTextParams {
     pub text: String,
+    pub role: ConversationTextRole,
+}
+
+#[derive(Debug, Clone, Copy, Default, PartialEq, Eq, Deserialize, Serialize, JsonSchema, TS)]
+#[serde(rename_all = "snake_case")]
+#[ts(rename_all = "snake_case")]
+pub enum ConversationTextRole {
+    #[default]
+    User,
+    Developer,
 }
 
 /// Persistent thread-settings overrides that can be applied before user input or