Skip setting 'Content-Type' on file upload, when multiple bodies are supported

Describe the bug
Form-style submissions to endpoints that support multiple different content types are missing the necessary "boundary" attribute on the Content-Type header.

POST https://.../items HTTP/1.1
Host: ...
Accept: */*
Accept-Encoding: gzip, deflate
Connection: keep-alive
User-Agent: python-httpx/0.25.2
Authorization: Bearer ...
Content-Type: multipart/form-data
Transfer-Encoding: chunked

Cause
https://github.com/openapi-generators/openapi-python-client/blob/660941dfe23a39ea9a22dd724e1db54afb130d08/openapi_python_client/templates/endpoint_module.py.jinja#L48-L63

In the template above, lines 60-62 have an "if content-type is not multipart/form-data" condition since httpx sets content type and multipart form boundary properly when files are provided. The same condition needs to be applied around line 54, which occurs when multiple body types are supported.

Comienza con openapi_python_client/templates/endpoint_module.py.jinja, especialmente con el manejo de Content-Type alrededor de las líneas 48-63 y la condición multipart existente. Verifica las requests generadas para endpoints que admiten varios tipos de body y considera completado el trabajo cuando las cargas multipart reciban una boundary de Content-Type sin sobrescribir el manejo de headers de httpx.