Use usd-exchange as standalone USD provider

[kellyguo11]

Description

Fix the repeated test_leapp_export_flow installation/runtime failure seen in the
Isaac Sim 6.0 CI jobs, including
job 91629341277.

PR #6806 made Isaac Lab's direct standalone USD dependencies platform-disjoint,
but two transitive dependency paths could still install both complete pxr
runtimes on x86_64:

• Isaac Sim's URDF converter requires usd-exchange.
• Newton's importers extra requires usd-core on x86_64.

This change uses usd-exchange==2.3.0 as the only standalone USD provider on all
supported platforms. It selects newton[sim] and declares Newton's non-USD
importer and mesh-processing requirements directly, preserving importer behavior
without reintroducing usd-core. The wheel override files and lockfile are kept in
sync, and regression tests cover the provider and Newton dependency contracts.

No new importer functionality is introduced; dependencies that were previously
transitive through newton[importers] are now explicit core requirements.

Type of change

• Bug fix (non-breaking change which fixes an issue)

Testing

• Confirmed the provider regression test fails against the previous usd-core
  metadata and passes with this change.
• Confirmed the Newton dependency regression test fails with
  newton[sim,importers] and passes with the explicit dependencies.
• 32 passed, 1 skipped across the focused CLI and wheel-metadata tests.
• uv lock --check passes; exported production requirements contain
  usd-exchange==2.3.0 and no usd-core.
• Changelog validation passes.
• uv run isaaclab -f passes twice.

Screenshots

Not applicable.

Checklist

• I have read and understood the contribution guidelines
• I have run the pre-commit checks with uv run isaaclab -f
• Documentation changes are not applicable to this dependency-only fix
• My changes generate no new warnings
• I have added tests that prove my fix is effective
• I have added a changelog fragment under source/isaaclab/changelog.d/
• My name already exists in CONTRIBUTORS.md