Remove CanvasSliderVisuals Update polling, refactor UpdateValue helper, and add tests

[keveleigh]

Fixes #521

Changes

• CanvasSliderVisuals:
  • Removed per-frame Update() polling.
  • Added Editor-only OnValidate() to reactively apply layout and handle updates when inspector properties (SliderDirection, etc.) change.
  • Added EnsureSliderEndpoints() helper to lazily resolve sliderStart/sliderEnd RectTransforms.
• Slider:
  • Added UpdateValue(float newValue, bool forceUpdate = false) helper to centralize value setting, clamping, step division snapping, and event firing.
  • Re-snapped and updated value when UseSliderStepDivisions or SliderStepDivisions properties are changed.
  • Updated Slider.OnValidate() to call UpdateValue(value, forceUpdate: true) so modifying the slider value in the Inspector during PlayMode immediately propagates OnValueUpdated.
• Tests:
  • Added EditMode tests in CanvasSliderVisualsTests.cs verifying endpoint caching, layout orientation changes, inspector OnValidate reactivity, and slider value updates.
  • Added PlayMode tests in CanvasSliderVisualsRuntimeTests.cs verifying runtime handle updates, direction changes, and step division snapping.

Verification

• EditMode tests: CanvasSliderVisualsTests (5/5 passed)
• PlayMode tests: CanvasSliderVisualsRuntimeTests (3/3 passed)