fix: resolve mutex conflicts in Instagram webhook specs (#12154)

This PR fixes flaky test failures in the Instagram webhook specs that
were caused by Redis mutex lock conflicts when
   tests ran in parallel.

 ### The Problem:
The InstagramEventsJob uses a Redis mutex with a key based on sender_id
and ig_account_id to prevent race
conditions. However, all test factories were using the same hardcoded
sender_id: 'Sender-id-1', causing multiple
test instances to compete for the same mutex lock when running in
parallel.

 ### The Solution:
- Updated all Instagram event factories to generate unique sender IDs
using SecureRandom.hex(4)
- Modified test stubs and expectations to work with dynamic sender IDs
instead of hardcoded values
- Ensured each test instance gets its own unique mutex key, eliminating
lock contention

spec/support/instagram_spec_helpers.rb

@@ -0,0 +1,20 @@
+module InstagramSpecHelpers
+  def create_instagram_contact_for_sender(sender_id, inbox)
+    contact = Contact.find_by(identifier: sender_id)
+    if contact.nil?
+      contact = create(:contact, identifier: sender_id, name: 'Jane Dae')
+      create(:contact_inbox, contact_id: contact.id, inbox_id: inbox.id, source_id: sender_id)
+    end
+    contact
+  end
+
+  def instagram_user_response_object_for(sender_id, account_id)
+    {
+      name: 'Jane',
+      id: sender_id,
+      account_id: account_id,
+      profile_pic: 'https://chatwoot-assets.local/sample.png',
+      username: 'some_user_name'
+    }
+  end
+end