fix: avoid 3d preview failure when OBJ cad asset is unreachable

[itisrohit]

Summary

Fixes a narrower 3d preview failure case: when a cad_component includes both model_obj_url and model_step_url, the old path preferred OBJ first.

If that OBJ asset is unreachable, the preview fails instead of falling back cleanly.

Before	After

Testing

bun test tests/jlcpcb-c2040-preview.test.ts

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[mohan-bee]

I don't find any issue with the 3d rendering for c2040,

[itisrohit]

You're right, the normal RP2040 3d render path looks fine.

This PR fixes a narrower case: when a cad_component includes both model_obj_url and model_step_url, the old 3d path preferred OBJ first. If that OBJ asset is unreachable, the render fails instead of falling back cleanly.

I've updated the PR title/body to reflect that narrower scope, and the regression test now targets that dual-source CAD case explicitly rather than the general c2040 render path.