feat(whatsapp): gate embedded signup inbox creation (#15046)

WhatsApp inbox creation now shows Embedded Signup for Chatwoot Cloud
accounts only when the new `whatsapp_embedded_signup_inbox_creation`
feature flag is enabled. Cloud accounts without the flag go directly to
manual WhatsApp Cloud API setup, while self-hosted installations with a
configured WhatsApp App ID retain their existing Embedded Signup flow.

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

config/features.yml

@@ -264,3 +264,7 @@
   display_name: WhatsApp Reconfigure
   enabled: false
   column: feature_flags_ext_1
+- name: whatsapp_embedded_signup_inbox_creation
+  display_name: WhatsApp Embedded Signup Inbox Creation
+  enabled: false
+  column: feature_flags_ext_1