[Python] KeyError in MapScalar with custom field name

[0x26res]

Describe the bug, including details regarding any error messages, version, and platform.

It is possible to customize the name of the value field for pa.MapType. I believe it's a niche feature used for compatibility with parquet and it doesn't change the semantic of the map type.

this feature causes a KeyError in MapScalar.as_py:

def test_bug_in_pyarrow():
    assert pa.scalar(
        [("foo", "bar")],
        pa.map_(
            pa.string(),
            pa.field("value", pa.string()),
        ),
    ).as_py() == [("foo", "bar")]

    with pytest.raises(KeyError, match=r"value"):
        pa.scalar(
            [("foo", "bar")],
            pa.map_(
                pa.string(),
                pa.field("map_value", pa.string()),
            ),
        ).as_py()

Component(s)

Python