Support attaching to existing realtime calls (#39876)

## Why

Clients that create and negotiate realtime calls themselves need Codex to join
those calls without creating another call or overwriting the client-owned
session configuration.

## What changed

- Add the `existingCall` transport to `thread/realtime/start`, accepting a
  client-provided `callId` and optional `realtimeSessionId`.
- Attach Codex over the call's sideband WebSocket without SDP negotiation or a
  session update, and reconnect that sideband while preserving transcript
  state.
- Default existing calls to realtime v1, support v1 and v3, and reject v2 or
  options that would reconfigure the client-owned session.
- Encode call IDs as a single URL path segment for v3 sideband connections.

## Testing

Add protocol, app-server, core, reconnection, authentication-header, and URL
encoding coverage for existing-call attachments.

GitOrigin-RevId: 2046b70566f98efa55381bb1e461ea3cc68256c5

codex-rs/app-server-protocol/schema/json/ClientRequest.json

@@ -4526,6 +4526,27 @@
           ],
           "title": "WebrtcThreadRealtimeStartTransport",
           "type": "object"
+        },
+        {
+          "properties": {
+            "callId": {
+              "description": "Identifier of a realtime call already created and negotiated by the client.",
+              "type": "string"
+            },
+            "type": {
+              "enum": [
+                "existingCall"
+              ],
+              "title": "ExistingCallThreadRealtimeStartTransportType",
+              "type": "string"
+            }
+          },
+          "required": [
+            "callId",
+            "type"
+          ],
+          "title": "ExistingCallThreadRealtimeStartTransport",
+          "type": "object"
         }
       ]
     },

codex-rs/app-server-protocol/schema/json/codex_app_server_protocol.schemas.json

@@ -21178,6 +21178,27 @@
             ],
             "title": "WebrtcThreadRealtimeStartTransport",
             "type": "object"
+          },
+          {
+            "properties": {
+              "callId": {
+                "description": "Identifier of a realtime call already created and negotiated by the client.",
+                "type": "string"
+              },
+              "type": {
+                "enum": [
+                  "existingCall"
+                ],
+                "title": "ExistingCallThreadRealtimeStartTransportType",
+                "type": "string"
+              }
+            },
+            "required": [
+              "callId",
+              "type"
+            ],
+            "title": "ExistingCallThreadRealtimeStartTransport",
+            "type": "object"
           }
         ]
       },

codex-rs/app-server-protocol/schema/json/codex_app_server_protocol.v2.schemas.json

@@ -18930,6 +18930,27 @@
           ],
           "title": "WebrtcThreadRealtimeStartTransport",
           "type": "object"
+        },
+        {
+          "properties": {
+            "callId": {
+              "description": "Identifier of a realtime call already created and negotiated by the client.",
+              "type": "string"
+            },
+            "type": {
+              "enum": [
+                "existingCall"
+              ],
+              "title": "ExistingCallThreadRealtimeStartTransportType",
+              "type": "string"
+            }
+          },
+          "required": [
+            "callId",
+            "type"
+          ],
+          "title": "ExistingCallThreadRealtimeStartTransport",
+          "type": "object"
         }
       ]
     },

codex-rs/app-server-protocol/schema/typescript/v2/ThreadRealtimeStartTransport.ts

@@ -10,4 +10,8 @@ export type ThreadRealtimeStartTransport = { "type": "websocket" } | { "type": "
  * SDP offer generated by a WebRTC RTCPeerConnection after configuring audio and the
  * realtime events data channel.
  */
-sdp: string, };
+sdp: string, } | { "type": "existingCall",
+/**
+ * Identifier of a realtime call already created and negotiated by the client.
+ */
+callId: string, };

codex-rs/app-server-protocol/src/protocol/v2/realtime.rs

@@ -155,6 +155,12 @@ pub enum ThreadRealtimeStartTransport {
         /// realtime events data channel.
         sdp: String,
     },
+    ExistingCall {
+        /// Identifier of a realtime call already created and negotiated by the client.
+        #[serde(rename = "callId")]
+        #[ts(rename = "callId")]
+        call_id: String,
+    },
 }
 
 /// EXPERIMENTAL - response for starting thread realtime.