Nodes: Ensure getBackgroundNode() and getEnvironmentNode() only return nodes.

[Mugen87]

Related issue: #30111

Description

This makes sure the backgroundNode and environmentNode are treated a bit more strictly than before since they only allow node input now. So something like below has no effect anymore:

scene.backgroundNode = new THREE.Color( 0x222222 );

It must be:

scene.backgroundNode = color( 0x222222 );

or

scene.background = new THREE.Color( 0x222222 );

[github-actions]

📦 Bundle size

Full ESM build, minified and gzipped.

	Before	After	Diff
WebGL	339.4 79.05	339.4 79.05	+0 B +0 B
WebGPU	486.48 134.97	486.67 135.01	+183 B +39 B
WebGPU Nodes	485.95 134.87	486.13 134.91	+183 B +36 B

🌳 Bundle size after tree-shaking

Minimal build including a renderer, camera, empty scene, and dependencies.

	Before	After	Diff
WebGL	465.3 112.11	465.3 112.11	+0 B +0 B
WebGPU	555.58 150.43	555.76 150.47	+183 B +40 B
WebGPU Nodes	511.65 140.19	511.83 140.23	+183 B +40 B