fix: preserve explicit assignee for bot inbox conversations

app/models/conversation.rb

@@ -296,6 +296,8 @@ class Conversation < ApplicationRecord
   end
 
   def assign_connected_agent_bot
+    return if assignee_id.present?
+
     self.assignee = nil
     self.assignee_agent_bot = inbox.agent_bot
   end

spec/models/conversation_spec.rb

@@ -663,6 +663,14 @@ RSpec.describe Conversation do
       expect(conversation.assignee).to be_nil
     end
 
+    it 'preserves explicit human assignee' do
+      agent = create(:user, account: bot_inbox.inbox.account)
+      conversation = create(:conversation, inbox: bot_inbox.inbox, assignee: agent)
+
+      expect(conversation.assignee).to eq(agent)
+      expect(conversation.assignee_agent_bot).to be_nil
+    end
+
     context 'with campaigns' do
       let(:user) { create(:user, account: bot_inbox.inbox.account) }