fix: Use contact_id instead of sender_id for Instagram message locks (#12841)

Previously, the lock key for Instagram used sender_id, which for echo
messages (outgoing) would be the account's own ID. This caused all
outgoing messages to compete for the same lock, creating a bottleneck
during bulk messaging.

The fix introduces contact_instagram_id method that correctly identifies
the contact's ID regardless of message direction:
- For echo messages (outgoing): uses recipient.id (the contact)
- For incoming messages: uses sender.id (the contact)

This ensures each conversation has a unique lock, allowing parallel
processing of webhooks while maintaining race condition protection
within individual conversations.

Fixes lock acquisition errors in Sidekiq when processing bulk Instagram
messages.

Fixes
https://linear.app/chatwoot/issue/CW-5931/p0-mutexapplicationjoblockacquisitionerror-failed-to-acquire-lock-for

## Type of change

- [x] Bug fix (non-breaking change which fixes an issue)

app/jobs/webhooks/instagram_events_job.rb

@@ -8,7 +8,7 @@ class Webhooks::InstagramEventsJob < MutexApplicationJob
   def perform(entries)
     @entries = entries
 
-    key = format(::Redis::Alfred::IG_MESSAGE_MUTEX, sender_id: sender_id, ig_account_id: ig_account_id)
+    key = format(::Redis::Alfred::IG_MESSAGE_MUTEX, sender_id: contact_instagram_id, ig_account_id: ig_account_id)
     with_lock(key) do
       process_entries(entries)
     end
@@ -77,6 +77,23 @@ class Webhooks::InstagramEventsJob < MutexApplicationJob
     @entries&.first&.dig(:id)
   end
 
+  def contact_instagram_id
+    entry = @entries&.first
+    return nil unless entry
+
+    # Handle both messaging and standby arrays
+    messaging = (entry[:messaging].presence || entry[:standby] || []).first
+    return nil unless messaging
+
+    # For echo messages (outgoing from our account), use recipient's ID (the contact)
+    # For incoming messages (from contact), use sender's ID (the contact)
+    if messaging.dig(:message, :is_echo)
+      messaging.dig(:recipient, :id)
+    else
+      messaging.dig(:sender, :id)
+    end
+  end
+
   def sender_id
     @entries&.dig(0, :messaging, 0, :sender, :id)
   end