fix: align Nous Portal model slugs with OpenRouter naming (#3253)

Nous Portal now passes through OpenRouter model names and routes from
there. Update the static fallback model list and auxiliary client default
to use OpenRouter-format slugs (provider/model) instead of bare names.

- _PROVIDER_MODELS['nous']: full OpenRouter catalog
- _NOUS_MODEL: google/gemini-3-flash-preview (was gemini-3-flash)
- Updated 4 test assertions for the new default model name

tests/agent/test_auxiliary_client.py

@@ -461,7 +461,7 @@ class TestGetTextAuxiliaryClient:
              patch("agent.auxiliary_client.OpenAI") as mock_openai:
             mock_nous.return_value = {"access_token": "nous-tok"}
             client, model = get_text_auxiliary_client()
-        assert model == "gemini-3-flash"
+        assert model == "google/gemini-3-flash-preview"
 
     def test_custom_endpoint_over_codex(self, monkeypatch, codex_auth_dir):
         monkeypatch.setenv("OPENAI_BASE_URL", "http://localhost:1234/v1")
@@ -693,7 +693,7 @@ class TestVisionClientFallback:
              patch("agent.auxiliary_client.OpenAI"):
             mock_nous.return_value = {"access_token": "nous-tok"}
             client, model = get_vision_auxiliary_client()
-        assert model == "gemini-3-flash"
+        assert model == "google/gemini-3-flash-preview"
         assert client is not None
 
     def test_vision_forced_main_uses_custom_endpoint(self, monkeypatch):
@@ -789,7 +789,7 @@ class TestResolveForcedProvider:
              patch("agent.auxiliary_client.OpenAI"):
             mock_nous.return_value = {"access_token": "nous-tok"}
             client, model = _resolve_forced_provider("nous")
-        assert model == "gemini-3-flash"
+        assert model == "google/gemini-3-flash-preview"
         assert client is not None
 
     def test_forced_nous_not_configured(self, monkeypatch):

tests/test_provider_parity.py

@@ -556,7 +556,7 @@ class TestAuxiliaryClientProviderPriority:
         with patch("agent.auxiliary_client._read_nous_auth", return_value={"access_token": "nous-tok"}), \
              patch("agent.auxiliary_client.OpenAI") as mock:
             client, model = get_text_auxiliary_client()
-        assert model == "gemini-3-flash"
+        assert model == "google/gemini-3-flash-preview"
 
     def test_custom_endpoint_when_no_nous(self, monkeypatch):
         monkeypatch.delenv("OPENROUTER_API_KEY", raising=False)