fix(shadcn): use native button for select trigger

[cocojojo5213]

Reasons for making this change

The shadcn single select trigger is implemented as a custom button-like element. Switching it to a native <button type="button"> gives it browser-managed button and disabled semantics while preserving keyboard activation.

This also adds regression coverage for tab navigation, aria-expanded / aria-haspopup, and disabled trigger behavior.

fixes #4764

Checklist

• I'm updating documentation
  • I've checked the rendering of the Markdown text I've added
• I'm adding or updating code
  • I've added and/or updated tests. I updated the shadcn snapshots and ran the package plus repo-level build checks listed below.
  • I've updated docs if needed
  • I've updated the changelog with a description of the PR
• I'm adding a new feature
  • I've updated the playground with an example use of the feature

Validation run:

• npm --workspace @rjsf/shadcn test -- SelectWidget.test.tsx
• npm --workspace @rjsf/shadcn run test:update
• npm --workspace @rjsf/shadcn test
• npm --workspace @rjsf/shadcn run cs-check
• npm --workspace @rjsf/shadcn run lint
• npm --workspace @rjsf/shadcn run build:ts
• npm --workspace @rjsf/shadcn run build
• npx nx run-many --target=build --exclude=@rjsf/docs --parallel=2
• git diff --check