[material_ui] Add SemanticsRole.slider to Slider

[hannah-hyj]

Adds SemanticsRole.slider, minValue, and maxValue to Slider semantics.

issue: flutter/flutter#184030

blocked by flutter/flutter#184029

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[gemini-code-assist]

Code Review

This pull request adds SemanticsRole.slider and min/max value bounds to the Slider semantics configuration, updating both the implementation and corresponding tests. Feedback suggests accessing the RenderObject's own min and max properties directly rather than through _state.widget to improve encapsulation.

[gemini-code-assist]

Instead of accessing the widget's properties directly via _state.widget.min and _state.widget.max, it is more idiomatic to use the RenderObject's own min and max properties. This respects the encapsulation of the RenderObject and ensures consistency with how other properties are accessed.

    config.minValue = min.toString();
    config.maxValue = max.toString();