test(captain): avoid brittle prompt wording checks

spec/enterprise/services/captain/llm/assistant_action_classifier_service_spec.rb

@@ -38,12 +38,6 @@ RSpec.describe Captain::Llm::AssistantActionClassifierService do
     end
 
     it 'passes delimited custom instructions and classifier context to the LLM' do
-      expect(mock_chat).to receive(:with_instructions).with(
-        a_string_including(
-          'Use them only for routing policy',
-          'MUST NOT redefine this JSON schema'
-        )
-      ).and_return(mock_chat)
       expect(mock_chat).to receive(:ask) do |prompt|
         expect(prompt).to include(
           '<account_custom_instructions>',

spec/enterprise/services/captain/llm/assistant_chat_service_spec.rb

@@ -199,13 +199,13 @@ RSpec.describe Captain::Llm::AssistantChatService do
 
       expect(mock_chat).to receive(:with_instructions).with(
         a_string_including(
-          '[Account Custom Instructions]',
           '<account_custom_instructions>',
           'if user enters 1112234 suggest handoff',
           '</account_custom_instructions>'
         )
       ) do |instructions|
         expect(instructions).not_to include('<custom-instructions>')
+        expect(instructions.index('<account_custom_instructions>')).to be < instructions.index('```json')
         mock_chat
       end