feat(whatsapp): unify embedded signup feature gating (#15106)

WhatsApp embedded signup now uses
`whatsapp_embedded_signup_inbox_creation` as the single Chatwoot Cloud
rollout gate for inbox creation, proactive reconfiguration, and
disconnected inbox reauthorization. The authorization endpoint enforces
the same gate, so the UI and backend remain consistent.

Self-hosted installations keep their existing behavior.

## Things to know

- This reuses the existing feature flag; there is no migration or schema
change.
- The feature is shown as “WhatsApp Embedded Signup Flow” in feature
management.
- `whatsapp_reconfigure` remains visible and honored for self-hosted
proactive reconfiguration to preserve existing accounts. It can be
deprecated after the self-hosted dependency is removed or migrated.

## How to test

1. On Chatwoot Cloud, enable `whatsapp_embedded_signup_inbox_creation`
for an account.
2. Confirm that new WhatsApp inbox creation, proactive reconfiguration,
and disconnected inbox reauthorization are available.
3. Disable the flag and confirm those entry points are hidden and
authorization requests are rejected.
4. On self-hosted, confirm proactive reconfiguration remains controlled
by the existing `whatsapp_reconfigure` account setting.

---------

Co-authored-by: Muhsin <12408980+muhsin-k@users.noreply.github.com>

config/features.yml

@@ -265,6 +265,6 @@
   enabled: false
   column: feature_flags_ext_1
 - name: whatsapp_embedded_signup_inbox_creation
-  display_name: WhatsApp Embedded Signup Inbox Creation
+  display_name: WhatsApp Embedded Signup Flow
   enabled: false
   column: feature_flags_ext_1