feat: add support for user message events from Codex

Add sendUserMessage() method to ApiSessionClient and CodexSession to handle
user_message events from Codex. Updates CodexConversionResult type and
convertCodexEvent() to process user_message payloads, with test coverage.

cli/src/codex/session.ts

@@ -45,6 +45,10 @@ export class CodexSession extends AgentSessionBase<EnhancedMode> {
         this.client.sendCodexMessage(message);
     };
 
+    sendUserMessage = (text: string): void => {
+        this.client.sendUserMessage(text);
+    };
+
     sendSessionEvent = (event: Parameters<ApiSessionClient['sendSessionEvent']>[0]): void => {
         this.client.sendSessionEvent(event);
     };