WhatsApp Business app Coexistence inboxes currently see the manual
migration prompt even though they cannot move to the generic manual
setup flow without losing the supported Coexistence path.
This change waits for WhatsApp health data and shows manual transfer
only when Meta confirms the number is not connected to the WhatsApp
Business app. Other eligible Embedded Signup inboxes continue to see the
migration prompt.
### Things to know
- This is frontend-only gating; the migration API behavior is unchanged.
- The prompt remains hidden while the WhatsApp Business app state is
unknown or unavailable.
### How to test
1. Enable `whatsapp_manual_transfer` for an account with an Embedded
Signup WhatsApp inbox.
2. Open an inbox whose health response reports `is_on_biz_app: true`;
confirm the migration banner and dialog are unavailable.
3. Open an inbox whose health response reports `is_on_biz_app: false`;
confirm the migration banner appears and opens the existing manual
migration dialog.
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Co-authored-by: Muhsin <12408980+muhsin-k@users.noreply.github.com>
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