feat: Add renderCustomTrigger prop to Select

[taheramr]

Adds a public renderCustomTrigger prop on Select that lets consumers fully replace the default trigger element while Select retains ownership of the dropdown lifecycle, focus return, ARIA wiring, and form-field integration.

The SelectProps.CustomTriggerProps props bag exposes:

• triggerRef — attach to the focusable element so Select can return focus on close
• isOpen — current dropdown state
• onClick — toggles the dropdown open/closed
• ariaProps — { id, 'aria-expanded', 'aria-labelledby', 'aria-describedby', 'aria-required' } to spread on the focusable element

The consumer is responsible for rendering aria-haspopup="listbox" on the focusable element themselves (fixed value, not in the bag).

When renderCustomTrigger is provided, the placeholder prop is not consumed by Select — the consumer renders their own empty state.

Test utilities:

• New findCustomTrigger() returns the wrapper around the consumer's custom trigger; null for default Select.
• Existing findTrigger() is unchanged for default Select; returns null when renderCustomTrigger is provided.

Backward compatible: default Select behavior is unchanged when the new prop is not provided.

Refs: AWSUI-61998

Description

Related links, issue #, if available: n/a

How has this been tested?

Review checklist

The following items are to be evaluated by the author(s) and the reviewer(s).

Correctness

• Changes include appropriate documentation updates.
• Changes are backward-compatible if not indicated, see CONTRIBUTING.md.
• Changes do not include unsupported browser features, see CONTRIBUTING.md.
• Changes were manually tested for accessibility, see accessibility guidelines.

Security

• If the code handles URLs: all URLs are validated through the checkSafeUrl function.

Testing

• Changes are covered with new/existing unit tests?
• Changes are covered with new/existing integration tests?

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