Skip obsolete MuJoCo fields when constructing named indexers

[sylvesterkaczmarek]

Fixes #525. Also addresses the same obsolete-field lookup failure described in #509.

Changes

MuJoCo releases can remove model/data attributes while locally generated field-size metadata still mentions them. Skip attributes that are absent from the runtime object, just as the existing named-indexing logic already skips non-array and structured fields.

Add tests that inject stale MjModel and MjData metadata, verify that missing fields are ignored, and confirm that present fields remain accessible by name.

Verification

• dm_control/mujoco/index_test.py: 692 passed, including both new regression tests.
• The tests used generated MuJoCo binding files copied locally for runtime validation. Those generated bindings are not included in this PR.
• No changes to simulation stepping, memory layout or metadata generation.