Reshape arrays rather than setting shape on them.

[shubhxho]

Fixes #540.

Problem

NumPy 2.5 deprecated assigning to ndarray.shape (release note, numpy#29536).

Axis.convert_key_item uses it on every named index built from a list or array of names, so ordinary named indexing emits a DeprecationWarning under NumPy >= 2.5. Two test helpers do the same.

With -W error::DeprecationWarning on NumPy 2.5.2 before the change:

8 failed, 678 passed

Fix

Use reshape instead. The semantics are the same at all three sites: the arrays are freshly built by np.array/np.arange and contiguous, so no copy is involved and no caller observes a difference.

Testing

$ python -W error::DeprecationWarning -m pytest dm_control/mujoco/index_test.py
686 passed

$ pytest dm_control/mjcf dm_control/mujoco
1741 passed, 11 skipped

The one unrelated failure on my machine, MujocoEngineTest::testRenderFlagOverridesAreNotPersistent, fails identically without this change (headless GL).

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