fix: validate identifier hash on widget contact update (#14884)

## Description

The widget contact update endpoint (`PATCH /api/v1/widget/contact`)
applied updates that set an `identifier` without validating the
`identifier_hash`, unlike `set_user`, which already does. This change
runs the same validation on the update path whenever an `identifier` is
supplied, keeping identity validation consistent across the widget
contact endpoints.

Anonymous updates that don't pass an identifier (prechat
name/email/phone and custom attributes) keep working unchanged,
including on inboxes with mandatory identity validation.

Fixes https://linear.app/chatwoot/issue/CW-7118


---------

Co-authored-by: Shivam Mishra <scm.mymail@gmail.com>
Co-authored-by: Sivin Varghese <64252451+iamsivin@users.noreply.github.com>

app/controllers/api/v1/widget/contacts_controller.rb

@@ -2,6 +2,7 @@ class Api::V1::Widget::ContactsController < Api::V1::Widget::BaseController
   include WidgetHelper
 
   before_action :validate_hmac, only: [:set_user]
+  before_action :validate_hmac_for_identified_update, only: [:update]
 
   def show; end
 
@@ -46,6 +47,16 @@ class Api::V1::Widget::ContactsController < Api::V1::Widget::BaseController
     @contact.identifier.present? && @contact.identifier != permitted_params[:identifier]
   end
 
+  # The plain update endpoint is also used for anonymous prechat updates
+  # (name/email/phone/custom_attributes with no identifier), which must keep
+  # working on hmac_mandatory inboxes. Only the identity-binding path, where an
+  # identifier is supplied and the contact can be rebound, requires HMAC.
+  def validate_hmac_for_identified_update
+    return if params[:identifier].blank?
+
+    validate_hmac
+  end
+
   def validate_hmac
     return unless should_verify_hmac?