Adds storage support for WhatsApp business-scoped user identifiers
received from Meta Cloud API and Twilio WhatsApp webhooks. The change
keeps existing phone-based behavior intact, stores BSUID and parent
BSUID values as additional `contact_inboxes.source_id` rows for the same
contact, and allows BSUID-only inbound messages to create contacts,
conversations, and messages without requiring a phone number.
Related: https://github.com/chatwoot/chatwoot/issues/13837
**What changed**
- Extended WhatsApp source ID validation to accept regular BSUID and
parent BSUID formats.
- For Meta Cloud API, stores phone, `user_id`, and `parent_user_id`
identifiers as contact inbox source IDs when they are present.
- For Twilio WhatsApp, stores phone, `ExternalUserId`, and
`ParentExternalUserId` identifiers as contact inbox source IDs while
preserving the existing `whatsapp:` Twilio source ID shape.
- Supports BSUID-only inbound messages by creating a contact, contact
inbox, conversation, and message even when the phone number is missing.
- Links phone-first and later BSUID-only messages to the same contact
when the first payload contains both phone and BSUID.
- Stores WhatsApp usernames in contact `additional_attributes`, matching
existing social channel patterns.
- Keeps existing phone-based outbound and new-conversation behavior
unchanged for this milestone.
**How to test**
1. Send a Meta Cloud webhook payload with both `wa_id` and `user_id`.
2. Verify Chatwoot creates or finds the phone `contact_inbox` and also
creates a BSUID `contact_inbox` for the same contact.
3. Send a later Meta Cloud payload for the same user with only `user_id`
/ `from_user_id`.
4. Verify Chatwoot finds the BSUID `contact_inbox` and creates the
inbound message without requiring a phone number.
5. Send a Twilio WhatsApp webhook with `From: whatsapp:+E164`,
`ExternalUserId`, and optionally `ParentExternalUserId`.
6. Verify Chatwoot stores the Twilio phone and BSUID identifiers as
`whatsapp:`-prefixed source IDs for the same contact.
7. Send a Twilio WhatsApp webhook where `From` is `whatsapp:<BSUID>` and
there is no phone number.
8. Verify Chatwoot creates the contact, contact inbox, conversation, and
message without a phone number.
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Co-authored-by: Muhsin <12408980+muhsin-k@users.noreply.github.com>
