fix: use config models when empty models list provided

When the trigger simulation API receives an empty models list ([]),
it now correctly falls back to enabled models from config instead
of running with no models.

Changes:
- Update condition to check for both None and empty list
- Add test case for empty models list behavior
- Update API documentation to clarify this behavior

All 28 integration tests pass.

API_REFERENCE.md

@@ -36,7 +36,7 @@ Trigger a new simulation job for a specified date range and models.
 |-------|------|----------|-------------|
 | `start_date` | string \| null | No | Start date in YYYY-MM-DD format. If `null`, enables resume mode (each model continues from its last completed date). Defaults to `null`. |
 | `end_date` | string | **Yes** | End date in YYYY-MM-DD format. **Required** - cannot be null or empty. |
-| `models` | array[string] | No | Model signatures to run. If omitted, uses all enabled models from server config. |
+| `models` | array[string] | No | Model signatures to run. If omitted or empty array, uses all enabled models from server config. |
 | `replace_existing` | boolean | No | If `false` (default), skips already-completed model-days (idempotent). If `true`, re-runs all dates even if previously completed. |
 
 **Response (200 OK):**

api/main.py

@@ -176,11 +176,11 @@ def create_app(
             with open(config_path, 'r') as f:
                 config = json.load(f)
 
-            if request.models is not None:
+            if request.models is not None and len(request.models) > 0:
                 # Use models from request (explicit override)
                 models_to_run = request.models
             else:
-                # Use enabled models from config
+                # Use enabled models from config (when models is None or empty list)
                 models_to_run = [
                     model["signature"]
                     for model in config.get("models", [])

tests/integration/test_api_endpoints.py

@@ -119,6 +119,18 @@ class TestSimulateTriggerEndpoint:
         data = response.json()
         assert data["total_model_days"] >= 1
 
+    def test_trigger_empty_models_uses_config(self, api_client):
+        """Should use enabled models from config when models is empty list."""
+        response = api_client.post("/simulate/trigger", json={
+            "start_date": "2025-01-16",
+            "end_date": "2025-01-16",
+            "models": []  # Empty list - should use enabled models from config
+        })
+
+        assert response.status_code == 200
+        data = response.json()
+        assert data["total_model_days"] >= 1
+
     def test_trigger_enforces_single_job_limit(self, api_client):
         """Should reject trigger when job already running."""
         # Create first job