Fixes issue #1276

Problem

When a requestBody lists multiple content types that all reference the same $ref schema, they all resolve to the same Python type. The generated _get_kwargs used a chain of if isinstance checks — but since
every branch matched, the last branch always won (typically multipart/form-data). It was impossible to make a JSON or form-urlencoded request.

# Generated (broken) — all three isinstance checks pass, multipart always wins
if isinstance(body, DnsCreate):
    _kwargs["json"] = body.to_dict()
    headers["Content-Type"] = "application/json"
if isinstance(body, DnsCreate):          # also matches!
    _kwargs["data"] = body.to_dict()
    headers["Content-Type"] = "application/x-www-form-urlencoded"
if isinstance(body, DnsCreate):          # also matches!
    _kwargs["files"] = body.to_multipart()
    headers["Content-Type"] = "multipart/form-data"

Fix

When duplicate Python types are detected across bodies, a body_content_type: str parameter is emitted (defaulting to the first content type in the spec), and dispatch uses if/elif string comparisons instead of
isinstance checks.

 # Generated (fixed)
 def _get_kwargs(
     *,
     body: DnsCreate | Unset = UNSET,
     body_content_type: str = "application/json",
 ) -> dict[str, Any]:
     ...
     if body_content_type == "application/json":
         _kwargs["json"] = body.to_dict()
         headers["Content-Type"] = "application/json"
     elif body_content_type == "application/x-www-form-urlencoded":
         _kwargs["data"] = body.to_dict()
         headers["Content-Type"] = "application/x-www-form-urlencoded"
     elif body_content_type == "multipart/form-data":
         _kwargs["files"] = body.to_multipart()
         headers["Content-Type"] = "multipart/form-data"

Endpoints with distinct body types (the existing behaviour) are unaffected.

Changes

• parser/openapi.py — adds bodies_with_content_type_dispatch (detects duplicate types) and unique_body_types (deduplicates for type annotations) properties to Endpoint
• templates/endpoint_macros.py.jinja — adds body_content_type to arguments and kwargs macros when needed
• templates/endpoint_module.py.jinja — uses if/elif string dispatch when bodies_with_content_type_dispatch is true
• end_to_end_tests/functional_tests/generated_code_execution/test_endpoint_bodies.py — functional tests verifying correct dispatch for JSON, form-urlencoded, and multipart