Set the branch header for the SDK realtime functions (#2128)

Author: matt-aitken

packages/core/src/v3/apiClient/index.ts

@@ -1016,11 +1016,15 @@ export class ApiClient {
   }
 
   #getRealtimeHeaders() {
-    const headers: Record<string, string> = {
+    let headers: Record<string, string> = {
       Authorization: `Bearer ${this.accessToken}`,
       "trigger-version": VERSION,
     };
 
+    if (this.previewBranch) {
+      headers["x-trigger-branch"] = this.previewBranch;
+    }
+
     return headers;
   }
 }