Fix EDT freezes when opening embedded DevTools and on theme change

[Copilot]

Summary

• Fix EDT freezes caused by:
  • Synchronous JxBrowser Engine initialization when opening embedded DevTools.
  • Blocking ExecutorService lifecycle on theme change (JDK 21 ExecutorService.close() awaits termination).

Changes

• EmbeddedJxBrowser.openEmbeddedTab:
  • Avoid initializing JxBrowser Engine on the EDT; show “installation in progress” UI until async init completes.
  • This removes the fallback compareAndSet(null, getEngine()) path that could call Engine.newInstance(...) on EDT and freeze it.
• FlutterInitializer.sendThemeChangedEvent:
  • Use shared AppScheduledExecutor (AppExecutorUtil) instead of creating/closing a new executor per event.
  • Remove try-with-resources that triggers ExecutorService.close() on EDT with JDK 21.

Notes

• Added comments explaining why Engine initialization must not occur on EDT and how JDK 21 changed ExecutorService.close().
• Included a lightweight regression test to ensure openEmbeddedTab returns quickly on EDT when the engine is not yet available.

Fixes

• Fixes IDE Freezing and slowing down a lot, I get error balloons a lot of times #8394
• Fixes 🚀 Performance freeze in EDT caused by ExecutorService blocking in Flutter plugin #8478

Testing

• Unit: EmbeddedJxBrowserTest.openEmbeddedTab_returnsQuicklyOnEdtWhenEngineMissing ensures the call returns promptly on EDT (no sync engine init).
• Manual:
  • Switch IDE themes repeatedly; no UI freeze.
  • Open DevTools (embedded) after clean startup and after JxBrowser installation completes; no EDT freeze, proper “installing...” UX during setup.

[google-cla]

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[AlexV525]

Apparently, Google's CLA is not ready for the Copilot yet. :)