fix(sitemap): clear iconcolor/labelcolor/valuecolor when an SSE event omits them - #1345

Summary

• OpenHABWidget.apply(event:) merged SSE color updates with if let, so it could only set a color, never clear one. When a widget's color condition (e.g. iconcolor=[==ON="orange"]) stopped matching, the server's SSE event for that widget simply omits the color field instead of sending an empty string — so the stale color stuck around forever, until a full sitemap reload rebuilt the widget from scratch.
• Regressed in Migrate sitemap page updates from long-polling to SSE #1168 (3.4.6 → 3.4.7), which moved sitemap updates from long-polling (always a full widget snapshot) to incremental SSE events. Reported via TestFlight feedback: a Switch item's iconcolor/labelcolor/valuecolor turned orange correctly on ON, but never reverted on OFF without a manual reload.
• Colors are now recomputed as a group whenever an event carries a state change (event.state or event.enrichedItem present) — an omitted color defaults to "" instead of being left untouched. Non-state events (icon reload, visibility-only) still leave colors alone, since the server never touches them there.

Test plan

• xcodebuild -workspace openHAB.xcworkspace -scheme openHAB build succeeds
• Added colorClearsOnStateChangeAwayFromMatchingCondition and nonStateEventLeavesColorsUntouched to OpenHABWidgetEventApplicationTests, both pass
• Full OpenHABCoreTests suite passes (-testPlan openHABTests -only-testing:OpenHABCoreTests)

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