docs and tests for on_primitive

[ChristopherBiscardi]

Objective

Add additional tests and docs for the on_primitive hook introduced in #22907

Solution

Tests currently only test failure conditions such as

• 0, 2, or more Meshes
• 0, 2, or more Primitives
• 0, 2, or more buffers

Which is the most important place to notify users early, considering the additional complexity incurred by following the gltf crate's types here.

Testing

cargo test -p bevy_gltf

[Smerom]

Based on your description, did you intend to add tests for the 2+ case? I'm only seeing one for the 0 case. Otherwise looks good!

[ChristopherBiscardi]

@Smerom I don't currently plan to. The 0 test case exercises the same code paths that 2+ would, including failing the same if conditions.