fix(browser): parenthesize document fallback for CoreBrowserService

Nullish coalescing binds tighter than the conditional operator, so the
previous expression was parsed as (this._document ?? windowDefined) ?
window.document : null. When this._document was set (e.g. parent
ownerDocument or documentOverride), the ternary still returned
window.document instead of the configured document.

CoreBrowserService.mainDocument is used by AccessibilityManager, viewport
styles, and decoration renderers; CharSizeService already received
this._document directly. Add parentheses so the fallback applies only when
_document is nullish.

Co-authored-by: Cursor <cursoragent@cursor.com>

src/browser/CoreBrowserTerminal.ts

@@ -505,7 +505,7 @@ export class CoreBrowserTerminal extends CoreTerminal implements ITerminal {
       this.textarea,
       parent.ownerDocument.defaultView ?? window,
       // Force unsafe null in node.js environment for tests
-      this._document ?? (typeof window !== 'undefined') ? window.document : null as any
+      this._document ?? ((typeof window !== 'undefined') ? window.document : null as any)
     ));
     this._instantiationService.setService(ICoreBrowserService, this._coreBrowserService);