fix(vscode,ui): stop postMessage crash when opening chat in Cursor (#2335)

VS Code webviews delete window.parent, so ChatContainer's settings-sync
effect threw TypeError on chat open. Also harden the webview bridge when
acquireVsCodeApi() returns undefined and SSE panel disposal races.

Co-authored-by: Cursor Agent <cursoragent@cursor.com>
Co-authored-by: Serhii Dziupin <makeittech@users.noreply.github.com>

packages/vscode/src/SessionEditorPanelProvider.ts

@@ -437,7 +437,8 @@ export class SessionEditorPanelProvider {
         headers: this._buildSseHeaders(headers),
         signal: controller.signal,
         onChunk: (chunk) => {
-          entry.panel.webview.postMessage({ type: 'api:sse:chunk', streamId, chunk });
+          // Panel may be disposed before SSE callbacks fire.
+          entry.panel?.webview?.postMessage({ type: 'api:sse:chunk', streamId, chunk });
         },
       });
 
@@ -445,12 +446,12 @@ export class SessionEditorPanelProvider {
 
       start.run
         .then(() => {
-          entry.panel.webview.postMessage({ type: 'api:sse:end', streamId });
+          entry.panel?.webview?.postMessage({ type: 'api:sse:end', streamId });
         })
         .catch((error) => {
           if (!controller.signal.aborted) {
             const messageText = error instanceof Error ? error.message : String(error);
-            entry.panel.webview.postMessage({ type: 'api:sse:end', streamId, error: messageText });
+            entry.panel?.webview?.postMessage({ type: 'api:sse:end', streamId, error: messageText });
           }
         })
         .finally(() => {