Add endpoint parameter to linspace

[AKnassa]

Proposed changes

Fixes #4152.

mx.linspace always includes the stop value. numpy, jax and the array API all
let the caller drop it and space the samples over the half-open interval
[start, stop) instead. This adds the same option:

mx.linspace(0, 10, 5)                  # [0, 2.5, 5, 7.5, 10]
mx.linspace(0, 10, 5, endpoint=False)  # [0, 2, 4, 6, 8]   (matches numpy)

The implementation is the one-line change it looks like: divide by num instead
of num - 1 when the endpoint is excluded.

endpoint defaults to true, so existing behaviour is unchanged.

Placement of the argument

I put endpoint after dtype and before the stream, both in the C++ signature
and in the Python binding, so the stream stays last as it is everywhere else in
the API. That does shift the position of StreamOrDevice in the C++ signature,
but no in-tree caller passes the stream positionally to linspace, and the
Python stream argument is keyword-only in practice. Happy to move it if you
would rather it went elsewhere — the array API puts endpoint keyword-only
after dtype/device, while numpy has it fourth positional, so there is no
single obvious answer.

Testing

• test_linspace_endpoint in python/tests/test_ops.py, checked against numpy
  for num in {0, 1, 2, 5, 50}, for negative ranges, and with an integer
  dtype. It fails before this change and passes after.
• Extended the existing test linspace case in tests/ops_tests.cpp, since the
  C++ signature changed, including the num == 1 and num == 0 edges.
• Full Python suite: 820 tests, no new failures.
• Full C++ suite: 247 cases, all pass.
• Docstring updated; the generated .pyi picks the new signature up on build.

Built CPU-only (MLX_BUILD_METAL=OFF); the GPU suite was not run on my machine.

Checklist

• 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)