Allow for BaseModel fields to be extracted from any index of the fields list.

fastapi/dependencies/utils.py

@@ -210,13 +210,16 @@ def get_flat_dependant(
 
 
 def _get_flat_fields_from_params(fields: List[ModelField]) -> List[ModelField]:
-    if not fields:
-        return fields
-    first_field = fields[0]
-    if len(fields) == 1 and lenient_issubclass(first_field.type_, BaseModel):
-        fields_to_extract = get_cached_model_fields(first_field.type_)
-        return fields_to_extract
-    return fields
+    extracted_fields = []
+
+    for field in fields:
+        if lenient_issubclass(field.type_, BaseModel):
+            model_fields = get_cached_model_fields(field.type_)
+            extracted_fields.extend(model_fields)
+        else:
+            extracted_fields.append(field)
+
+    return extracted_fields
 
 
 def get_flat_params(dependant: Dependant) -> List[ModelField]: