Fix race condition by making window.complete() async and awaiting it

- window.complete() was fire-and-forget, causing execution-complete to arrive before console messages
- This caused sandbox to unregister before console message responses arrived
- Made complete() async and await sendRuntimeMessage()
- SandboxedIframe wrapper now awaits window.complete()
- Ensures all messages are processed before cleanup/unregister
- Fixes 30-second timeout on javascript_repl

packages/web-ui/src/components/SandboxedIframe.ts

@@ -339,7 +339,7 @@ export class SandboxIframe extends LitElement {
 					}
 				}
 
-				window.complete();
+				await window.complete();
 			} catch (error) {
 				console.error(error?.stack || error?.message || String(error));
 
@@ -352,7 +352,7 @@ export class SandboxIframe extends LitElement {
 					}
 				}
 
-				window.complete({
+				await window.complete({
 					message: error?.message || String(error),
 					stack: error?.stack || new Error().stack
 				});