Fix dispose leaks: register CursorBlinkStateManager + _pausedResizeTask

Three related disposable-registration gaps that leak xterm Terminal
instances when the host application unmounts a Terminal. Evidence +
full writeup at juspay/kolu#606.

1. addon-webgl/WebglRenderer._cursorBlinkStateManager was declared
   `= new MutableDisposable()` without the `this._register(...)`
   wrapper that every sibling MutableDisposable in the class uses.
   CursorBlinkStateManager runs a setInterval for the cursor blink
   and its `dispose()` correctly clears it — but dispose() was never
   called because the disposable wasn't registered. The live timer
   kept the renderer (and thus the Terminal) alive past host
   unmount.

2. common/TaskQueue.DebouncedIdleTask had no `dispose()` method.
   Added one that clears the internal IdleTaskQueue (cancelling any
   pending idle callback).

3. browser/services/RenderService._pausedResizeTask (a
   DebouncedIdleTask) was not registered. Wrapped with
   `this._register(...)` so RenderService.dispose propagates.

Verified in a downstream app (kolu) with a repro that toggles Focus
vs Canvas layout modes:

- Pre-fix: 24/28 disposed xterm Terminals retained past host unmount
  + forced GC.
- Post-fix: 0/28 retained.

Retention chain (before fix):

    DOMTimer → ScheduledAction → V8Function
      → CursorBlinkStateManager._renderCallback → WebglRenderer
      → Terminal

addons/addon-webgl/src/WebglRenderer.ts

@@ -30,7 +30,7 @@ import { createRenderDimensions } from 'browser/renderer/shared/RendererUtils';
 
 export class WebglRenderer extends Disposable implements IRenderer {
   private _renderLayers: IRenderLayer[];
-  private _cursorBlinkStateManager: MutableDisposable<CursorBlinkStateManager> = new MutableDisposable();
+  private _cursorBlinkStateManager: MutableDisposable<CursorBlinkStateManager> = this._register(new MutableDisposable());
   private _textBlinkStateManager: TextBlinkStateManager;
   private _charAtlasDisposable = this._register(new MutableDisposable());
   private _charAtlas: ITextureAtlas | undefined;