Fix RangeIndex.linspace with num=1

[sanand0]

Description

RangeIndex.linspace handles num=1 like numpy.linspace

Checklist

• Closes Should RangeIndex.linspace handle num=1 like numpy.linspace? #11397
• Tests added
• No user visible changes
• No new functions/methods

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Codex Prompts

codex --yolo --model gpt-5.5 --config model_reasoning_effort=medium

I've cloned https://github.com/pydata/xarray/ here. Fix https://github.com/pydata/xarray/issues/11397 - i.e.
MINIMALLY and ELEGANTLY modify `xarray/indexes/range_index.py` as a small maintainer-friendly change so that

- it doesn't increase the number of code lines and
- works when num = 1, matching the NumPy behaviour, i.e. `np.linspace(0, 1, num=1)` returns `array([0.])`

I believe adding `and num > 1` to the `if` condition in `xarray/indexes/range_index.py` would fix this.

```python
if endpoint and num > 1:
    stop += (stop - start) / (num - 1)
```

Begin by writing a regression test like the below (but write it consistent with the xarray test style) to verify that the bug is fixed:

```python
def test_range_index_linspace_num_1():
    index = RangeIndex.linspace(0.0, 1.0, num=1, dim="x")
    assert_array_equal(index.transform.generate_coords()["x"], np.array([0.0]))

def test_range_index_linspace_num_1_endpoint_false():
    index = RangeIndex.linspace(0.0, 1.0, num=1, endpoint=False, dim="x")
    assert_array_equal(index.transform.generate_coords()["x"], np.array([0.0]))
```

Run and test, make sure the tests fail (before the fix) only because of the bug and pass after the fix.

---

Is it possible to fold the test into an existing test function so that the test code becomes more elegant and maintainable? If so, do that.

---

Is further simplification possible AND desirable?

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