Fix cfpb-select multiple toggle state

[arpitjain099]

When cfpb-select switches from multiple back to single mode, it can keep more than one option checked.

This normalizes the selection down to one option (the most recent checked value) and updates the single-select label to match. I also added a regression test that covers the toggle path.

Tested locally with:

• npx -y @yarnpkg/cli-dist@4.16.0 eslint packages/cfpb-design-system/src/elements/cfpb-select/index.js packages/cfpb-design-system/src/elements/cfpb-select/index.spec.js
• npx -y @yarnpkg/cli-dist@4.16.0 vitest run packages/cfpb-design-system/src/elements/cfpb-select/index.spec.js packages/cfpb-design-system/src/elements/cfpb-select/multiple-select-event-proxy.spec.js
• npx -y @yarnpkg/cli-dist@4.16.0 vite build --config vite.config.packages.js

Fixes #2609