Adjust unknown color mode handling in ZHA lights

[epenet]

Proposed change

Spotted in #162276

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[home-assistant]

Hey there @dmulcahey, @Adminiuga, @puddly, @TheJulianJES, mind taking a look at this pull request as it has been labeled with an integration (zha) you are listed as a code owner for? Thanks!

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[TheJulianJES]

We'll also switch ZHA to never return None. Not sure if it even does, but it's still typed like that.

[Copilot]

Pull request overview

This PR adjusts the ZHA light component to properly handle unknown color modes following the changes in PR #162276, which removes the legacy fallback for reporting color_mode as None. The ZHA light integration now explicitly returns ColorMode.UNKNOWN instead of None when the underlying entity's color mode is not set.

Changes:

• Changed the return type of the color_mode property from ColorMode | None to ColorMode
• Modified the fallback behavior to return ColorMode.UNKNOWN instead of None when the entity's color mode is not available