fix: skip AutoAssignment bulk loop when no agents are online (#14500)

## Description

When an inbox has `enable_auto_assignment` and `assignment_v2` enabled
but no agents are currently online,
`AutoAssignment::AssignmentService#perform_bulk_assignment` still loaded
up to 100 unassigned conversations and iterated each one, calling
`inbox.available_agents` per conversation. Each call hits Redis presence
lookups that return empty, no conversations get assigned, and the loop
finishes having done only wasted work.

For a busy inbox with a long unassigned backlog and offline agents, this
is hundreds of Redis ops per job, multiplied by every
`AutoAssignment::AssignmentJob` enqueue from the per-save handler. The
pressure is significant when inbound volume is high.

This adds a single early-return guard: if
`inbox.available_agents.empty?`, return `0` immediately. Existing
semantics are preserved (jobs are still enqueued on conversation events;
they just exit cheaply when there is no one to assign to).

## Type of change

- [x] Performance improvement (non-breaking change)

## Test coverage

- [x] Added specs

app/services/auto_assignment/assignment_service.rb

@@ -5,12 +5,14 @@ class AutoAssignment::AssignmentService
     return 0 unless inbox.auto_assignment_v2_enabled?
     return 0 unless inbox.enable_auto_assignment?
 
-    assigned_count = 0
+    conversations = unassigned_conversations(limit).to_a
+    return 0 if conversations.empty?
+    return 0 if inbox.available_agents.empty?
 
-    unassigned_conversations(limit).each do |conversation|
+    assigned_count = 0
+    conversations.each do |conversation|
       assigned_count += 1 if perform_for_conversation(conversation)
     end
-
     assigned_count
   end
 

spec/services/auto_assignment/assignment_service_spec.rb

@@ -56,6 +56,19 @@ RSpec.describe AutoAssignment::AssignmentService do
         expect(conversation.reload.assignee).to be_nil
       end
 
+      it 'short-circuits without iterating conversations when no agents are online' do
+        3.times do
+          conv = create(:conversation, inbox: inbox, status: 'open')
+          conv.update!(assignee_id: nil)
+        end
+        allow(OnlineStatusTracker).to receive(:get_available_users).and_return({})
+
+        expect(service).not_to receive(:perform_for_conversation)
+
+        assigned_count = service.perform_bulk_assignment(limit: 10)
+        expect(assigned_count).to eq(0)
+      end
+
       it 'respects the limit parameter' do
         3.times do
           conv = create(:conversation, inbox: inbox, status: 'open')