Add contrast checker for pseudo-state color panels

[dhruvikpatel18]

Closes #78305

What?

Adds contrast-ratio checking to all interactive pseudo-state color panels (:hover, :focus, :focus-visible, :active) in the block inspector.
When a user sets a low-contrast color combination for a pseudo-state, a warning notice now appears, the same notice already shown for the default state.
Also fixes a false-positive warning that could appear when the user was editing the default state and physically hovered the block in the canvas while the inspector was open.

Why?

PR #76491 added support for styling blocks per interactive pseudo-state, which is great for theming. However, the existing contrast checker (BlockColorContrastChecker) works by reading colors via window.getComputedStyle on the live canvas DOM element.
In the default state this works correctly: the computed styles reflect the block's current colors.
In a pseudo-state (:hover, :focus, etc.) the canvas element is always in its resting state. The pseudo-state CSS rules never apply to the DOM unless the user physically hovers/focuses the element, which they won't be doing while typing in the color picker. So getComputedStyle silently returns the default-state colors and the contrast check is effectively skipped, exactly the bug described issue.
The false-positive case was the inverse: if the user happened to hover the canvas while on the default panel, the DOM checker would briefly read the hover-state CSS and emit a spurious warning.

Testing Instructions

1. Create a new page / post.
2. Add a Button block.
3. In the block inspector, confirm the State dropdown shows "Default".
4. Set a good-contrast default: black text (#000000) on white background (#ffffff). Confirm no contrast warning appears.
5. Switch the State dropdown to ":hover".
6. Change the background color to a light yellow (#ffffcc).
7. Leave the text color unset (it should inherit the default black #000000 from step 4). Confirm no contrast warning, black on light yellow passes AA.
8. Now change the default text color to white (#ffffff) while still on :hover (or set it on the default panel first). Switch back to :hover.
9. Set the :hover text color explicitly to black (#000000). Confirm the warning disappears (passes AA again).
10. Switch back to the Default state. Physically hover your mouse over the button in the canvas. Confirm no spurious contrast warning appears.

Screenshots or screencast

Screen.Recording.2026-05-15.at.4.17.29.PM.mov

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Co-authored-by: dhruvikpatel18 <dhruvik18@git.wordpress.org>
Co-authored-by: talldan <talldanwp@git.wordpress.org>
Co-authored-by: juanfra <juanfra@git.wordpress.org>

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