Fix ReferenceError caused by undefined `isValidMessage` and `MESSAGE_ROLES`
in the buildApiPayload function within api.js.
Changes:
- Add missing `isValidMessage` import from utils.js
- Add missing `MESSAGE_ROLES` import from playground constants
- Consolidate duplicate `formatMessageForAPI` import
- Clean up import statements organization
Resolves: ReferenceError: isValidMessage is not defined at buildApiPayload (api.js:39:13)
