fix: assign outbound voice call conversation to the calling agent (#14906)

Outbound voice calls were being auto-assigned to the wrong agent. When
an agent placed an outbound call, the conversation was created without
an assignee, so inboxes with auto-assignment enabled would round-robin
it to a different agent instead of keeping it with the person who
actually made the call. This made it hard to tell which agent was on an
active call.

## What changed

- Set the calling agent as the conversation's assignee when creating an
outbound voice call conversation.
- This prevents the generic auto-assignment handler from treating the
conversation as unassigned and reassigning it.
- Added specs covering the assignment, including a regression case with
inbox auto-assignment enabled.

**Note:** this applies to newly placed calls; it does not retroactively
fix conversations that were already mis-assigned.

---------

Co-authored-by: Tanmay Deep Sharma <tanmaydeepsharma21@gmail.com>
Co-authored-by: Tanmay Deep Sharma <32020192+tds-1@users.noreply.github.com>

enterprise/app/controllers/api/v1/accounts/whatsapp_calls_controller.rb

@@ -106,9 +106,14 @@ class Api::V1::Accounts::WhatsappCallsController < Api::V1::Accounts::BaseContro
 
   def create_outbound_call
     contact_phone = @conversation.contact.phone_number.delete('+')
+    # Claim for the caller only if unassigned at trigger time (before the round-trip); wins over auto-assignment.
+    claim_for_caller = @conversation.assignee_id.nil?
+
     result = provider_service.initiate_call(contact_phone, params[:sdp_offer])
     provider_call_id = result.dig('calls', 0, 'id') || result['call_id']
 
+    @conversation.with_lock { @conversation.update!(assignee: Current.user) } if claim_for_caller
+
     Current.account.calls.create!(
       provider: :whatsapp, inbox: @conversation.inbox, conversation: @conversation, contact: @conversation.contact,
       provider_call_id: provider_call_id, direction: :outgoing, status: 'ringing',