🐛 Flatten multiple Pydantic models for parameters in path, query, header and cookie

From #12199 onwards, Pydantic models are supported for query, cookie and
header parameters. When one parameter is present, the model is flattened
in the OpenAPI spec, but when multiple are defined, they aren't.

This is confusing, and results in a confusing OpenAPI spec. Since these
arguments are used in flattened form anyway, it makes more sense to
flatten all of them.

fastapi/dependencies/utils.py

@@ -212,11 +212,14 @@ 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
+    result = []
+    for field in fields:
+        if lenient_issubclass(field.type_, BaseModel):
+            fields_to_extract = get_cached_model_fields(field.type_)
+            result.extend(fields_to_extract)
+        else:
+            result.append(field)
+    return result
 
 
 def get_flat_params(dependant: Dependant) -> List[ModelField]: