fix: Apply filter for inbox when the user is an admin (#11197)

Optimization #11183 missed a condition where the inbox_id filter is
manually passed. Due to the previous change, the inbox filter was being
discarded for admins, although it continued to work correctly for
agents.

This PR includes a fix for that specific case and adds a spec to
explicitly test it.

spec/finders/conversation_finder_spec.rb

@@ -57,6 +57,16 @@ describe ConversationFinder do
 
         expect(result[:conversations].map(&:id)).not_to include(restricted_conversation.id)
       end
+
+      it 'returns only the conversations from the inbox if inbox_id filter is passed' do
+        conversation = create(:conversation, account: account, inbox_id: inbox.id)
+        params = { inbox_id: restricted_inbox.id }
+        result = described_class.new(admin, params).perform
+
+        conversation_ids = result[:conversations].map(&:id)
+        expect(conversation_ids).not_to include(conversation.id)
+        expect(conversation_ids).to include(restricted_conversation.id)
+      end
     end
 
     context 'with assignee_type all' do