Fixes #8999

Since v2.20.0 (#8950), the base GraphicsDevice constructor calls updateClientRect(), which unconditionally calls this.canvas.getBoundingClientRect() outside of Web Workers. This broke the documented headless path — constructing a NullGraphicsDevice with a mock canvas (e.g. { id: 'mock' }, as @playcanvas/react does for SSR) now throws, which broke static site generation on developer.playcanvas.com.

Changes:

• GraphicsDevice#updateClientRect now uses a capability check (typeof this.canvas.getBoundingClientRect === 'function') instead of the platform.worker environment check. When the method is missing (OffscreenCanvas in workers, mock canvases in headless environments), it falls back to this.canvas.width ?? 0 / height ?? 0, so a bare mock object yields clientRect = { width: 0, height: 0 } rather than throwing or producing NaN.
• Worker behavior is unchanged (OffscreenCanvas has no getBoundingClientRect either), and real DOM canvases still get a valid clientRect at construction time, preserving the intent of Fix camera.screenToWorld returning NaN when called in initialize #8950.
• Added unit tests covering a mock canvas without getBoundingClientRect, a mock with width/height, and a DOM canvas using getBoundingClientRect.