[codex] Fix realtime v1 websocket compatibility (#23771)

## Why

Realtime v1 websocket sessions now expect a slightly different boundary
shape for text input, completed input transcripts, and connection
headers. Codex was still using the older shape, so some v1 text appends
could be rejected before the existing conversation flow could handle
them.

## What changed

- Send v1 user text items with `input_text` content
- Accept v1 turn-marked input transcript events as completed transcripts
- Add the v1 alpha header only for v1 realtime sessions
- Cover the outbound text shape, transcript parsing, and versioned
headers

## Test plan

- `cargo test -p codex-api endpoint::realtime_websocket::methods::tests`
- `cargo test -p codex-core quicksilver_alpha_header`

codex-rs/codex-api/src/endpoint/realtime_websocket/methods.rs

@@ -993,6 +993,22 @@ mod tests {
         );
     }
 
+    #[test]
+    fn parse_v1_input_transcript_turn_marked_event() {
+        let payload = json!({
+            "type": "conversation.input_transcript.turn_marked",
+            "transcript": "hello realtime"
+        })
+        .to_string();
+
+        assert_eq!(
+            parse_realtime_event(payload.as_str(), RealtimeEventParser::V1),
+            Some(RealtimeEvent::InputTranscriptDone(RealtimeTranscriptDone {
+                text: "hello realtime".to_string(),
+            }))
+        );
+    }
+
     #[test]
     fn parse_output_transcript_delta_event() {
         let payload = json!({
@@ -1581,6 +1597,10 @@ mod tests {
                 .expect("text");
             let third_json: Value = serde_json::from_str(&third).expect("json");
             assert_eq!(third_json["type"], "conversation.item.create");
+            assert_eq!(
+                third_json["item"]["content"][0]["type"],
+                Value::String("input_text".to_string())
+            );
             assert_eq!(third_json["item"]["content"][0]["text"], "hello agent");
 
             let fourth = ws

codex-rs/codex-api/src/endpoint/realtime_websocket/methods_v1.rs

@@ -21,7 +21,7 @@ pub(super) fn conversation_item_create_message(text: String) -> RealtimeOutbound
             r#type: ConversationItemType::Message,
             role: ConversationRole::User,
             content: vec![ConversationItemContent {
-                r#type: ConversationContentType::Text,
+                r#type: ConversationContentType::InputText,
                 text,
             }],
         }),

codex-rs/codex-api/src/endpoint/realtime_websocket/protocol.rs

@@ -199,7 +199,6 @@ pub(super) struct ConversationItemContent {
 #[derive(Debug, Clone, Copy, Serialize)]
 #[serde(rename_all = "snake_case")]
 pub(super) enum ConversationContentType {
-    Text,
     InputText,
 }
 

codex-rs/codex-api/src/endpoint/realtime_websocket/protocol_v1.rs

@@ -43,7 +43,8 @@ pub(super) fn parse_realtime_event_v1(payload: &str) -> Option<RealtimeEvent> {
         | "conversation.item.input_audio_transcription.delta" => {
             parse_transcript_delta_event(&parsed, "delta").map(RealtimeEvent::InputTranscriptDelta)
         }
-        "conversation.item.input_audio_transcription.completed" => {
+        "conversation.input_transcript.turn_marked"
+        | "conversation.item.input_audio_transcription.completed" => {
             parse_transcript_done_event(&parsed, "transcript")
                 .map(RealtimeEvent::InputTranscriptDone)
         }