fix(python,asyncio): multipart form data serialization (#19302)

* fix: object serialization for multipart requests

This PR is essentially
<https://github.com/OpenAPITools/openapi-generator/pull/18140> but for
the asyncio client.

* fix: int serialization for multipart requests

urllib3 handles serializing ints in post params (ref 1), while asyncio
explicitly does not (ref 2).

ref 1: <9316764e90/src/urllib3/filepost.py (L75-L76)>
ref 2: <https://github.com/aio-libs/aiohttp/issues/920>

* test: new fake multipart endpoint with files and body

* test: regression test for stringified body params

* fix: mypy tweak

* fix: FILES regeneration

* feat: object, int serialization for multipart reqs

Extends previous commits (and #18140) to cover the python-pydantic-v1
client as well.

* fix: use async with in test

* test: regression test for pydantic-v1-aiohttp

* test: add regression test to pydantic-v1

Also brings the second test in line with the first, patching
`urllib3.PoolManager.urlopen`

samples/openapi3/client/petstore/python-pydantic-v1-aiohttp/.openapi-generator/FILES

@@ -112,6 +112,7 @@ docs/TestObjectForMultipartRequestsRequestMarker.md
 docs/Tiger.md
 docs/UnnamedDictWithAdditionalModelListProperties.md
 docs/UnnamedDictWithAdditionalStringListProperties.md
+docs/UploadFileWithAdditionalPropertiesRequestObject.md
 docs/User.md
 docs/UserApi.md
 docs/WithNestedOneOf.md
@@ -233,6 +234,7 @@ petstore_api/models/test_object_for_multipart_requests_request_marker.py
 petstore_api/models/tiger.py
 petstore_api/models/unnamed_dict_with_additional_model_list_properties.py
 petstore_api/models/unnamed_dict_with_additional_string_list_properties.py
+petstore_api/models/upload_file_with_additional_properties_request_object.py
 petstore_api/models/user.py
 petstore_api/models/with_nested_one_of.py
 petstore_api/py.typed