Align UsdUVTexture wrap modes with USD specification

[pixelsandpointers]

Closes #2899

The wrap mode enum values for ND_UsdUVTexture_23 did not match those used by UsdUVTexture in OpenUSD, making round-trip interchange lossy. This PR aligns the names, defaults, and fallback behavior with the USD specification.

Changes

libraries/bxdf/usd_preview_surface.mtlx

• Renamed enum values periodic → repeat and black is now explicitly included (was previously named constant)
• Changed the default for wrapS/wrapT from periodic to useMetaData, matching the USD default

source/MaterialXRender/ImageHandler.h

• Renamed AddressMode enum values: CONSTANT → BLACK, PERIODIC → REPEAT to match USD terminology

GL / Metal / Slang texture handlers

• Updated all references to the renamed enum values
• Changed the default (unspecified) address mode fallback from REPEAT (GL: GL_REPEAT, Metal: MTLSamplerAddressModeRepeat) to BLACK (GL: GL_CLAMP_TO_BORDER, Metal: MTLSamplerAddressModeClampToBorderColor), matching USD's fallback from useMetaData when no metadata is present

Not yet addressed

useMetaData is now accepted as the default enum value in the nodedef, but reading wrap modes from texture file metadata is not yet implemented — a follow-up is noted with a TODO (#2899) comment in both the nodedef and the AddressMode enum.

[pixelsandpointers]

I think this is up for review @ld-kerley and @jstone-lucasfilm.

I am not entirely sure if all the changes I introduced were necessary (still fairly new to contributing to MtlX). Therefore, I am also uncertain if the versioning code should still target 1.39, or 1.40.

The useMetaData is not targeted in this PR. I added fallthrough and fallback cases to black. I may need to read a bit more about how OpenUSD handles useMetaData to propose a solution.

Happy for feedback and comments.

[jstone-lucasfilm]

@pixelsandpointers I haven't yet done a full review, but on my first read-through, it looks like you're renaming the address modes of all texture nodes, including the core image node in MaterialX, when you probably mean to restrict this change to the UsdUVTexture node alone.

[pixelsandpointers]

@jstone-lucasfilm I was not sure which files concretely touch what. I grepped for the ND_UsdUVTexture23 and applied the changes there first and later looked into the ImageHandler defining the enumerations. Afterwards, I followed all the old enum values in all downstream definitions and changed them to use the new ones..

Concretely, that means I need to revert which changes? I assume I need to revert the gltf definitions, right?