Adjust openrgb default color mode handling

[epenet]

Proposed change

Follow-up to #162276
Spotted in #162644

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[home-assistant]

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

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[Copilot]

Pull request overview

This PR adjusts the OpenRGB light entity’s default color_mode handling to align with recent core light validation changes (follow-up to #162276 / spotted in #162644), avoiding invalid/legacy None color mode handling and ensuring a valid fallback behavior.

Changes:

• Initialize color_mode with ColorMode.UNKNOWN instead of None.
• Update the fallback condition to treat ColorMode.UNKNOWN as the “unset” state and default to ColorMode.RGB.

[Copilot]

This change adjusts the fallback from None to ColorMode.UNKNOWN, which affects the initial/off-by-color/off-by-mode paths. There is currently no test asserting that an OpenRGB light starting in Off mode (or Off-by-color) reports valid supported_color_modes (i.e., not including ColorMode.UNKNOWN) and doesn’t trigger light color mode validation warnings. Add a test case covering these states to prevent regressions.