fix(urdf): improve collision-role and resolvable UDIM export

[MrHenriA]

Summary

I reproduced the two exporter problems reported in #709.

First, hidden or guide-purpose collision geometry could appear in the URDF visual output when Visualize Collisions was disabled. This produced visible collision boxes after the URDF was imported back into Isaac Sim.

Second, an encoded UDIM path could be written into the generated MTL file without resolving the concrete tiles or copying the texture files into the exported package.

What changed

• Pass the collision visualization setting into link conversion.
• Keep collision-only geometry out of visual output by default.
• Preserve visible collision-enabled geometry for visual and collision use.
• Normalize literal and percent-encoded UDIM tokens.
• Resolve and copy concrete supported UDIM tiles.
• Assign OBJ faces to concrete tile materials.
• Reject unsupported cross-tile faces.
• Avoid unresolved texture paths in generated MTL files.
• Add five focused regression tests.

Tests

The focused Isaac extension test run completed with:

• Ran 5 tests in 11.797s
• ISSUE709_EXTENSION_TESTS=PASS total=5 failures=0 errors=0

The tested commit and the submitted candidate have the same source tree.

Test cases

I validated the change with three separate NVIDIA assets:

• iW Hub using the workflow reported in Isaac Sim 6.0.1: USD-to-URDF export causes missing colors and large visible collision boxes #709.
• UR10e as an independent manipulator test case.
• JetBot as a separate mobile robot with more complicated presentation materials.

For each test case, I compared the source asset, stock reimport, and patched reimport.

Related work

PR #774 independently addresses the collision-visualization option reported in #709. This PR overlaps that narrow portion and additionally addresses resolvable UDIM export, texture packaging, per-tile material assignment, unsupported mapping
diagnostics, and three-asset visual validation.

Limits

NVIDIA documents that “there is no direct one-to-one mapping between USDs and URDFs” because USD can describe a broader set of scenes and robots than URDF.

The exporter documentation lists assumptions and constraints for supported source USD assets.

This patch improves collision-role handling and resolvable UDIM delivery within the supported exporter path. It does not claim lossless USD → URDF → USD conversion. Depending on the asset, materials, metadata, names, joints, sensors, and application-specific behavior may still change or be omitted.

The exported robot still requires structural and visual review.

Reference: NVIDIA Isaac Sim: Export URDF - Limitations

Visual validation

Addresses #709