ColorPicker: Add tests for ColorPicker Alpha slider

[im3dabasia]

What?

Closes #49344
Related comment: #49214 (comment)

Related PRs: #49214, #49698

This PR improves the unit tests for the ColorPicker component by adding test coverage for the Alpha slider and its corresponding input field when enableAlpha. Previously, tests covered most input fields (hex, HSL, RGB, and hue slider), but the Alpha slider had no dedicated tests.

Why?

The maintainers have emphasized that these tests are important for the editor. While previous tests validated various input interactions, they did not:

• Ensure that the Alpha slider updates correctly when the input field is changed.
• Cover enableAlpha={true}, leaving a gap in test coverage.

How?

• Added tests to check that the Alpha slider updates when the user types in the Alpha input field and when adjusting the slider.
• Verified that the correct RGBA/HSLA values are passed to onChange when changing transparency via both the input field and the slider.
• Used user-event for more accurate simulation of real user interactions.

Testing Instructions

1. Run the test by the following command:

npm run test:unit packages/components/src/color-picker/test/index.tsx

2. Check that the new test cases for the Alpha slider and input field pass.
3. Confirm that the test verifies the expected value in onChange when modifying alpha.
4. Review the test implementation to ensure coverage of both slider and input interactions.

Screenshot

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Co-authored-by: im3dabasia <[email protected]>
Co-authored-by: ciampo <[email protected]>

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