Support tooltip arrow color in display settings and extract findElement utility

[karngyan]

Summary

Two changes in this PR:

1. Support tooltipArrowColor in display settings

Adds a tooltipArrowColor property to DisplaySettings, allowing the tooltip arrow color to be customized via the display settings API. When switching away from tooltip display type (to modal, overlay, or inline), the arrow color is cleared along with other tooltip-specific properties.

• hasDisplayChanged detects arrow color changes so tooltips re-render when the color is updated
• applyDisplaySettings sets the color on the message properties when the display type is tooltip, and clears it for all other display types
• Full test coverage for setting, detecting changes, and clearing on display type switch

2. Extract shared findElement DOM utility

Consolidates duplicate element lookup logic into a single findElement utility in src/utilities/dom.ts. Both message-component-manager.ts (previously safelyFetchElement) and tooltip-position-manager.ts now use this shared function. The utility tries getElementById first, then falls back to querySelector, with a try/catch for invalid selectors.

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