fix: return cached dimensions after renderer dispose

[openwong2kim]

Both #5586 (mouse event path) and #5826 (decoration refresh path) crash on the same line:

public get dimensions(): IRenderDimensions { return this._renderer.value!.dimensions; }

After dispose(), the MutableDisposable clears its value, so _renderer.value is undefined and the non-null assertion throws TypeError: Cannot read properties of undefined (reading 'dimensions').

#5591 and #5827 fixed the two known crash paths with per-callsite guards (isDisposed / hasRenderer() checks). That works, but there are 20+ remaining unguarded .dimensions accesses across:

• CoreBrowserTerminal.ts — cursor positioning, overview ruler sizing (10+)
• AccessibilityManager.ts — live region sizing (4)
• BufferDecorationRenderer.ts — decoration element sizing (6+)
• MouseCoordsService.ts — mouse coordinate math (4)

Any async callback (rAF, setTimeout, intersection observer) that races dispose() on one of those paths will hit the same crash. Adding a guard at every callsite is whack-a-mole.

This PR fixes the getter itself — cache the last known IRenderDimensions on every read and return it after dispose. Stale values are harmless since the terminal is mid-teardown.

- public get dimensions(): IRenderDimensions { return this._renderer.value!.dimensions; }
+ private _cachedDimensions: IRenderDimensions = createRenderDimensions();
+ public get dimensions(): IRenderDimensions {
+   const renderer = this._renderer.value;
+   if (renderer) {
+     this._cachedDimensions = renderer.dimensions;
+   }
+   return this._cachedDimensions;
+ }

The per-callsite guards from #5591/#5827 can stay (they prevent unnecessary work on a disposed terminal) but are no longer load-bearing for crash safety.

Builds clean with tsgo -b ./tsconfig.all.json.