fix(auxiliary): honor api_mode in auxiliary client (#6800)

The auxiliary client always calls client.chat.completions.create(),
ignoring the api_mode config flag. This breaks codex-family models
(e.g. gpt-5.3-codex) on direct OpenAI API keys, which need the
/v1/responses endpoint.

Changes:
- Expand _resolve_task_provider_model to return api_mode (5-tuple)
- Read api_mode from auxiliary.{task}.api_mode config and env vars
  (AUXILIARY_{TASK}_API_MODE)
- Pass api_mode through _get_cached_client to resolve_provider_client
- Add _needs_codex_wrap/_wrap_if_needed helpers that wrap plain OpenAI
  clients in CodexAuxiliaryClient when api_mode=codex_responses or
  when auto-detection finds api.openai.com + codex model pattern
- Apply wrapping at all custom endpoint, named custom provider, and
  API-key provider return paths
- Update test mocks for the new 5-tuple return format

Users can now set:
  auxiliary:
    compression:
      model: gpt-5.3-codex
      base_url: https://api.openai.com/v1
      api_mode: codex_responses

Closes #6800

tests/agent/test_auxiliary_client.py

@@ -1136,7 +1136,7 @@ class TestCallLlmPaymentFallback:
         with patch("agent.auxiliary_client._get_cached_client",
                     return_value=(primary_client, "google/gemini-3-flash-preview")), \
              patch("agent.auxiliary_client._resolve_task_provider_model",
-                    return_value=("openrouter", "google/gemini-3-flash-preview", None, None)), \
+                    return_value=("openrouter", "google/gemini-3-flash-preview", None, None, None)), \
              patch("agent.auxiliary_client._try_payment_fallback",
                     return_value=(fallback_client, "gpt-5.2-codex", "openai-codex")) as mock_fb:
             result = call_llm(
@@ -1162,7 +1162,7 @@ class TestCallLlmPaymentFallback:
         with patch("agent.auxiliary_client._get_cached_client",
                     return_value=(primary_client, "google/gemini-3-flash-preview")), \
              patch("agent.auxiliary_client._resolve_task_provider_model",
-                    return_value=("openrouter", "google/gemini-3-flash-preview", None, None)):
+                    return_value=("openrouter", "google/gemini-3-flash-preview", None, None, None)):
             with pytest.raises(Exception, match="Internal Server Error"):
                 call_llm(
                     task="compression",
@@ -1179,7 +1179,7 @@ class TestCallLlmPaymentFallback:
         with patch("agent.auxiliary_client._get_cached_client",
                     return_value=(primary_client, "google/gemini-3-flash-preview")), \
              patch("agent.auxiliary_client._resolve_task_provider_model",
-                    return_value=("openrouter", "google/gemini-3-flash-preview", None, None)), \
+                    return_value=("openrouter", "google/gemini-3-flash-preview", None, None, None)), \
              patch("agent.auxiliary_client._try_payment_fallback",
                     return_value=(None, None, "")):
             with pytest.raises(Exception, match="insufficient credits"):