realtime: rename provider session ids (#20361)

## Summary

Codex is repurposing `session` to mean a thread group, so the realtime
provider session id should no longer use `session_id` / `sessionId` in
Codex-facing protocol payloads. This PR renames that provider-specific
field to `realtime_session_id` / `realtimeSessionId` and intentionally
breaks clients that still send the old field names.

## What Changed

- Renamed realtime provider session fields in `ConversationStartParams`,
`RealtimeConversationStartedEvent`, and `RealtimeEvent::SessionUpdated`.
- Renamed app-server v2 realtime request and notification fields to
`realtimeSessionId`.
- Removed legacy serde aliases for `session_id` / `sessionId`; clients
must send the new names.
- Propagated the rename through core realtime startup, app-server
adapters, codex-api websocket handling, and TUI realtime state.
- Regenerated app-server protocol schema/TypeScript outputs and updated
app-server README examples.
- Kept upstream Realtime API concepts unchanged: provider `session.id`
parsing and `x-session-id` headers still use the upstream wire names.

## Testing

- CI is running on the latest pushed commit.
- Earlier local verification on this PR:
  - `cargo test -p codex-protocol`
- `CODEX_SKIP_VENDORED_BWRAP=1 cargo test -p codex-core
realtime_conversation`
  - `cargo test -p codex-app-server-protocol`
- `CODEX_SKIP_VENDORED_BWRAP=1 cargo test -p codex-app-server
realtime_conversation`
- attempted `CODEX_SKIP_VENDORED_BWRAP=1 cargo test -p codex-tui` (local
linker bus error while linking the test binary)

---------

Co-authored-by: Codex <noreply@openai.com>

codex-rs/protocol/src/protocol.rs

@@ -163,7 +163,7 @@ pub struct ConversationStartParams {
     )]
     pub prompt: Option<Option<String>>,
     #[serde(skip_serializing_if = "Option::is_none")]
-    pub session_id: Option<String>,
+    pub realtime_session_id: Option<String>,
     #[serde(skip_serializing_if = "Option::is_none")]
     pub transport: Option<ConversationStartTransport>,
     #[serde(skip_serializing_if = "Option::is_none")]
@@ -366,7 +366,7 @@ pub struct RealtimeResponseDone {
 #[derive(Debug, Clone, Deserialize, Serialize, PartialEq, Eq, JsonSchema, TS)]
 pub enum RealtimeEvent {
     SessionUpdated {
-        session_id: String,
+        realtime_session_id: String,
         instructions: Option<String>,
     },
     InputAudioSpeechStarted(RealtimeInputAudioSpeechStarted),
@@ -1667,7 +1667,7 @@ pub enum RealtimeConversationVersion {
 
 #[derive(Debug, Clone, Deserialize, Serialize, PartialEq, JsonSchema, TS)]
 pub struct RealtimeConversationStartedEvent {
-    pub session_id: Option<String>,
+    pub realtime_session_id: Option<String>,
     pub version: RealtimeConversationVersion,
 }
 
@@ -4764,14 +4764,14 @@ mod tests {
         let start = Op::RealtimeConversationStart(ConversationStartParams {
             output_modality: RealtimeOutputModality::Audio,
             prompt: Some(Some("be helpful".to_string())),
-            session_id: Some("conv_1".to_string()),
+            realtime_session_id: Some("conv_1".to_string()),
             transport: None,
             voice: None,
         });
         let webrtc_start = Op::RealtimeConversationStart(ConversationStartParams {
             output_modality: RealtimeOutputModality::Audio,
             prompt: Some(Some("be helpful".to_string())),
-            session_id: Some("conv_1".to_string()),
+            realtime_session_id: Some("conv_1".to_string()),
             transport: Some(ConversationStartTransport::Webrtc {
                 sdp: "v=offer\r\n".to_string(),
             }),
@@ -4784,14 +4784,14 @@ mod tests {
         let default_prompt_start = Op::RealtimeConversationStart(ConversationStartParams {
             output_modality: RealtimeOutputModality::Audio,
             prompt: None,
-            session_id: None,
+            realtime_session_id: None,
             transport: None,
             voice: None,
         });
         let null_prompt_start = Op::RealtimeConversationStart(ConversationStartParams {
             output_modality: RealtimeOutputModality::Audio,
             prompt: Some(None),
-            session_id: None,
+            realtime_session_id: None,
             transport: None,
             voice: None,
         });
@@ -4803,7 +4803,7 @@ mod tests {
                 "type": "realtime_conversation_start",
                 "output_modality": "audio",
                 "prompt": "be helpful",
-                "session_id": "conv_1"
+                "realtime_session_id": "conv_1"
             })
         );
         assert_eq!(
@@ -4880,7 +4880,7 @@ mod tests {
                 "type": "realtime_conversation_start",
                 "output_modality": "audio",
                 "prompt": "be helpful",
-                "session_id": "conv_1",
+                "realtime_session_id": "conv_1",
                 "transport": {
                     "type": "webrtc",
                     "sdp": "v=offer\r\n"
@@ -4890,6 +4890,22 @@ mod tests {
         );
     }
 
+    #[test]
+    fn realtime_conversation_started_event_uses_realtime_session_id() {
+        let event = RealtimeConversationStartedEvent {
+            realtime_session_id: Some("conv_1".to_string()),
+            version: RealtimeConversationVersion::V2,
+        };
+
+        assert_eq!(
+            serde_json::to_value(&event).unwrap(),
+            json!({
+                "realtime_session_id": "conv_1",
+                "version": "v2"
+            })
+        );
+    }
+
     #[test]
     fn realtime_voice_list_is_stable() {
         assert_eq!(