fix(captain): always include instructions in Codex responses body

Codex endpoint retorna HTTP 400 "Instructions are required" quando o
campo vem ausente. Agora sempre incluímos o campo — string com espaço
quando não há system message no request.

Validado end-to-end: curl → /codex/v1/chat/completions → proxy traduz
→ Codex devolve streaming SSE → proxy agrega → JSON Chat Completions.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

enterprise/app/services/captain/codex/translator.rb

@@ -15,10 +15,12 @@ class Captain::Codex::Translator
     body = {
       model: chat_body['model'] || chat_body[:model],
       input: input,
+      # Codex exige o campo instructions sempre. Se não vier system message,
+      # usamos uma string com um espaço pra satisfazer o endpoint sem influenciar o comportamento.
+      instructions: instructions.presence || ' ',
       store: false,
       stream: true # Codex exige streaming sempre — o Client agrega.
     }
-    body[:instructions] = instructions if instructions
     body[:max_output_tokens] = chat_body['max_tokens'] if chat_body['max_tokens']
 
     tools = chat_body['tools'] || chat_body[:tools]

spec/enterprise/services/captain/codex/translator_spec.rb

@@ -130,6 +130,20 @@ RSpec.describe Captain::Codex::Translator do
 
       expect(result.keys).not_to include(:temperature, :top_p, :frequency_penalty)
     end
+
+    it 'always includes instructions (empty placeholder when no system message)' do
+      chat = {
+        'model' => 'gpt-5.4',
+        'messages' => [{ 'role' => 'user', 'content' => 'Oi' }]
+      }
+
+      result = described_class.chat_to_responses(chat)
+
+      # Codex exige o campo instructions mesmo sem system message.
+      expect(result).to have_key(:instructions)
+      expect(result[:instructions]).not_to be_nil
+      expect(result[:instructions]).not_to eq('')
+    end
   end
 
   describe '.responses_to_chat' do