Fix cellPart toolbar event listener memory leak

[Copilot]

Problem

A memory leak was occurring in notebook cell toolbars where event listeners were accumulating and not being properly disposed when cells were unrendered. The error stack trace pointed to:

at bge.D in ./file:/Users/cloudtest/vss/_work/1/s/src/vs/workbench/contrib/notebook/browser/view/cellParts/cellToolbars.ts:231:23

Root Cause

In CellTitleToolbarPart.setupChangeListeners(), event listeners for menu.onDidChange and toolbar.onDidChangeDropdownVisibility were being registered to the main disposable store using this._register() instead of the cell-specific disposable store.

This caused issues because:

• These listeners are created during didRenderCell for each cell rendering cycle
• When unrenderCell is called, it only clears this.cellDisposables
• The listeners remained in the main disposable store, never getting cleaned up
• Rapid cell creation/disposal cycles caused listener accumulation

Solution

Changed the event listener registration to use the cell-specific disposable store:

// Before (leaked)
this._register(menu.onDidChange(() => { ... }));
this._register(toolbar.onDidChangeDropdownVisibility(visible => { ... }));

// After (properly disposed)
this.cellDisposables.add(menu.onDidChange(() => { ... }));
this.cellDisposables.add(toolbar.onDidChangeDropdownVisibility(visible => { ... }));

This ensures the listeners are disposed when unrenderCell() calls this.cellDisposables.clear().

Testing

The fix targets the exact line mentioned in the error stack trace and follows the established pattern where cell-specific resources are managed by cellDisposables while component-level resources use the main disposable store.

Fixes #248531.

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