update 3D scene when an <extra-model> is removed

[diegoteran]

Fixes #5186.

Summary & Root Cause

When an <extra-model> child element was removed from <model-viewer> (e.g. via element.remove()), the DOM child change triggered a slotchange event, which invoked this[$updateSource](). However, because the primary model had already finished loading, this.loaded remained true. Inside [$updateSource](), an early return check (if (this.loaded || ...)) caused the update to bail out before invoking scene.setSource(). As a result, the removed <extra-model>'s 3D mesh was orphaned and remained rendered inside the Three.js scene graph. ### Changes Made

• model-viewer-base.ts:
  • Updated [$onSlotChange] to reset this[$loaded] = false; and this[$loadedTime] = 0; whenever the list of <extra-model> URLs or element count changes. This allows [$updateSource]() to proceed past the load guard and properly synchronize the Three.js scene graph with the DOM.
  • Added explicit handling for the edge case where all primary and extra models are removed (this.src == null && extraModels.length === 0), directly invoking this[$scene].reset() and clearing loaded state.
• ModelScene.ts:
  • Updated reset() to ensure this.extraUrls = []; is properly cleared during scene resets.
• extra-model-spec.ts:

Reference Issue