FORMS-25113: Show multiselect dropdown options as a scrollable visible list

[gaurav5826]

Problem

When a dropdown is configured as multiselect (type: string[]), the <select> element rendered with a fixed height that collapsed the list — making it difficult for users to see and pick from available options.

Solution

• Added a CSS rule for [multiple] attribute (instead of [multiple='multiple']) using field-sizing: content and height: auto so the multiselect renders as an expanded visible list
• Capped with max-height: 80px to prevent taking up too much vertical space
• Single-select dropdowns are unaffected

Test plan

• Multiselect dropdown renders as an expanded visible list
• Single-select dropdown behavior and appearance unchanged
• Existing styles remain intact

Fixes: FORMS-25113