Fix TypeScript implicit type errors in SearchableDropdown tests

[Copilot]

Fixed TS7018 compilation errors where TypeScript couldn't infer generic types from null and empty array literals in test props.

Changes

• Added explicit type annotation null as TestItem | null for selectedItem property in default test props
• Added explicit type annotation [] as TestItem[] for items property in empty items test case

const defaultProps = {
  // ...
  selectedItem: null as TestItem | null,  // was: null
  // ...
};

const propsWithEmptyItems = {
  // ...
  items: [] as TestItem[],  // was: []
};

This allows TypeScript to properly infer the generic type parameter T in SearchableDropdownProps<T>.

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