Fix disposable leak in TOCTree stylesheet creation

[Copilot]

The TOCTree constructor created a stylesheet element via createStyleSheet(container) but never tracked it for disposal, leaking DOM elements and memory.

Changes

• Pass DisposableStore to createStyleSheet() to track the stylesheet lifecycle
• Reuse single stylesheet instance in styleController factory instead of creating new ones
• Register the store with this.disposables for cleanup on tree disposal

Before:

styleController: id => new DefaultStyleController(domStylesheetsJs.createStyleSheet(container), id)

After:

const disposableStore = new DisposableStore();
const styleElement = domStylesheetsJs.createStyleSheet(container, undefined, disposableStore);
// ...
styleController: id => new DefaultStyleController(styleElement, id)
// ...
this.disposables.add(disposableStore);

Added test with ensureNoDisposablesAreLeakedInTestSuite() to verify fix.

Original prompt

Check for disposable leaks in tocTree and fix any that are there.

Created from VS Code.

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