[BUG] dcc.RangeSlider ignores allowCross=False and lets handles cross #3935

Context

dash 4.4.1 (editable source checkout)

• Reproduced at commit: 3d3a9f70f452fba4dc385db94590009ce44031fc
• OS: Ubuntu 24.04.4 LTS
• Browser: Google Chrome 151.0.7922.71

The bug

dcc.RangeSlider does not enforce allowCross=False. When the lower thumb is
dragged past the upper thumb, the dragged thumb crosses it and becomes the new
upper thumb.

For example, a slider starting at [5, 15] ends at [15, 18] after its lower
thumb is dragged toward 18. The returned array is still sorted, but the thumb
identity and resulting interval prove that crossing occurred despite being
explicitly disabled. Application callbacks therefore cannot rely on the
allowCross=False constraint.

Minimal reproducible example

from dash import Dash, Input, Output, dcc, html


app = Dash(__name__)
app.layout = html.Div(
    style={"margin": "80px auto", "width": "500px"},
    children=[
        dcc.RangeSlider(
            id="range-slider",
            min=0,
            max=20,
            step=1,
            value=[5, 15],
            allowCross=False,
            updatemode="drag",
            tooltip={"always_visible": True},
        ),
        html.Pre(id="range-output", style={"marginTop": "40px"}),
    ],
)


@app.callback(
    Output("range-output", "children"),
    Input("range-slider", "value"),
    Input("range-slider", "drag_value"),
)
def show_values(value, drag_value):
    return f"value={value}\ndrag_value={drag_value}"


if __name__ == "__main__":
    app.run()

Steps to reproduce

1. Run the example and open http://127.0.0.1:8050.
2. Drag the lower handle from 5 past the upper handle at 15, toward 18.
3. Observe that the dragged thumb passes the other thumb and the callback
   reports value=[15, 18] and drag_value=[15, 18].

Expected behavior

With allowCross=False, a handle should stop when it reaches its neighboring
handle. In the example above, dragging the lower handle toward 18 should
clamp both handles at 15, producing:

value=[15, 15]
drag_value=[15, 15]

The same no-cross constraint should apply to pointer, keyboard, Home/End, and
direct-input interactions, with both updatemode="drag" and
updatemode="mouseup".

Screenshots

The bug at commit 3d3a9f70f;