Closes #20723

Fix Dropdown showing validation states when disabled or readonly

Description

Prevents dropdown components (React and Web Components) from displaying invalid/warn states when in disabled or readonly mode, as users cannot interact with non-interactive components to fix validation issues.

Changelog

Changed

React Component (Dropdown.tsx):

• Added isInteractive flag (!readOnly && !disabled) to conditionally show validation states
• Introduced derived values showInvalid and showWarning based on interactivity
• Applied consistently across: CSS classes, ListBox props, validation icons, aria-describedby, and helper text rendering

Web Components (dropdown.ts):

• Changed invalid and warn properties from reflect: true to reflect: false
• Added manual attribute reflection in updated() method to control when attributes appear on host element
• Implemented same isInteractive logic in _classes getter and render() method
• Fixed shouldUpdate() to properly enable/disable child items when disabled property toggles
• Applied validation state logic to: CSS classes, helper text classes, validation icons, and helper message display

Testing / Reviewing

React Component:

1. Open Dropdown component in Storybook
2. Set invalid={true} and invalidText="Error message"
3. Verify invalid state is visible (red border, error icon, error text)
4. Set disabled={true}
5. Verify invalid state is hidden (no red border, no error icon, helper text shown instead)
6. Set disabled={false}
7. Verify invalid state returns
8. Repeat steps 4-7 with readOnly={true}
9. Test same flow with warn prop

Web Components:

1. Create dropdown with invalid and invalid-text="Error message"
2. Verify invalid state is visible
3. Inspect element - confirm invalid attribute exists on <cds-dropdown>
4. Set disabled attribute
5. Verify invalid state is hidden and invalid attribute is removed from host
6. Repeat with read-only attribute

Expected behavior:

• When disabled=true or readOnly=true: No validation classes, no validation icons, no validation text, no validation attributes on host element
• When interactive (default): Validation states display normally
• Helper text displays when non-interactive, validation text displays when interactive and invalid/warn

Previous behavior:

• Validation states (invalid/warn) were shown even when user couldn't interact with the component
• Created confusing UX where disabled/readonly fields showed errors users couldn't fix

Accessibility Impact

Improved:

• aria-describedby correctly associates helper text instead of validation text when non-interactive
• Validation attributes removed from host element when non-interactive, preventing screen readers from announcing errors on disabled/readonly fields
• Helper text remains available to assistive technologies for disabled/readonly states

PR Checklist

As the author of this PR, before marking ready for review, confirm you:

• Reviewed every line of the diff
• Wrote passing tests that cover this change
• Addressed any impact on accessibility (a11y)
• Tested for cross-browser consistency
• Validated that this code is ready for review and status checks should pass

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