Fix copy bug in mlx.core.asarray

[aaishwarymishra]

Please include a description of the problem or feature this PR is addressing. If there is a corresponding issue, include the issue #.

This pull request refines how the mx.asarray and mx.array functions handle the copy=False option when given an existing mx::array. Now, if the input array already matches the requested dtype, a copy is avoided as expected. If a dtype conversion would be needed, an error is raised if copy=False is specified. The test suite is updated to verify this improved behavior.

Improvements to array creation logic

• Updated create_array in python/src/convert.cpp to only raise an error for copy=False when a dtype conversion is required, otherwise returning the original array without copying.
• Removed redundant code handling for mx::array instances in the type dispatch, consolidating logic for clarity and correctness.

Test suite updates

• Changed test_asarray_copy and test_asarray in python/tests/test_array.py to confirm that no error is raised when copy=False is used with an array of the same dtype, and that errors are still raised if a dtype conversion is required without copying. [1] [2]## Proposed changes

Checklist

Put an x in the boxes that apply.

• I have read the CONTRIBUTING document
• I have run pre-commit run --all-files to format my code / installed pre-commit prior to committing changes
• I have added tests that prove my fix is effective or that my feature works
• I have updated the necessary documentation (if needed)

[zcbenz]

The reason we throw for copy=False is that, semantically it means the copy would be shallow:

b = array(1)
a = asarray(b, copy=False)
b += 1
assert(a == 2)

which is not true in MLX because we don't have in-place updates, and accepting copy=False would just confuse users.