feat: Add filtering to button dropdown

[avinashbot]

Description

Button dropdown action filtering implemented a similar way to select/multiselect filtering. Initial prototype, took a lot of wrangling after the basic Claude-generated starting point, and there's a little bit of cleaning up still left to do. Though it should be mostly bug-free.

Related links, issue #, if available: AWSUI-61960

How has this been tested?

Updated unit tests, a bit of manual accessibility testing, but more validation with other screen readers is needed.

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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