[MISC] Improve readability of entity and morph repr.

[duburcqa]

Description

Two related fixes to the artist repr:

1. repr_base brief no longer probes attributes (hasattr/getattr). Entities and indexed sub-objects own their _repr_brief. Heterogeneous entities now report their variant count (e.g. morph: 2 morph variants) instead of reading the ambiguous .morph property.
2. File morphs no longer dump inline documents. When an MJCF/URDF is built in memory, the whole serialized document is stored in file; FileMorph/USD.__repr_name__ previously printed it verbatim in both the header and the field listing. They now show a bounded identifier, e.g. file='<inline probe>' for <mujoco model="probe">, so the morph keeps its identity without dumping the document. The identifier comes from a single polymorphic Morph._identifier() that _get_morph_identifier (entity naming) also delegates to, replacing the per-type isinstance chain. get_robot_name now accepts inline URDF content (matching get_model_name). Also fixes USD repr rendering file twice.

Related Issue

Follow-up surfaced while reviewing #2798 (heterogeneous-morph .morph access).

Motivation and Context

repr of a heterogeneous entity silently collapsed to its first morph, and repr of an in-memory MJCF/URDF morph dumped the entire XML into the terminal.

How Has This Been / Can This Be Tested?

pytest tests/test_misc.py::test_repr_does_not_crash - one scene packs a plane, primitive, file-based MJCF, inline MJCF, heterogeneous rigid, and kinematic entity plus a camera, then renders every repr/_repr_brief path and sanity-checks that a file morph shows its path while an inline one shows its parsed model name rather than the document. Entity-naming tests (test_urdf_mjcf_names_from_file, test_auto_and_user_names) confirm the _identifier consolidation preserves names.

Checklist:

• I read the CONTRIBUTING document.
• I followed the Submitting Code Changes section of CONTRIBUTING document.
• I tagged the title correctly (including BUG FIX/FEATURE/MISC/BREAKING)
• I updated the documentation accordingly or no change is needed.
• I tested my changes and added instructions on how to test it for reviewers.
• I have added tests to cover my changes.
• All new and existing tests passed.

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