Generation for multiple content types for a single endpoint fails when content types expect identical models

首先，使用 issue 中的 OpenAPI 片段重現生成流程，並檢查產生的 endpoint _get_kwargs 函式。追蹤相同模型如何與每種請求內容類型建立關聯；當 JSON、form-urlencoded 和 multipart 請求仍可供選擇，而不是被最後一個分支覆寫時，即完成。

Describe the bug
Support for multiple content types for a single endpoint was added in #453 / #822. However, the implemented fix generates non-functional code if the content types expect the same model. For instance, take this Api Spec snippet:

paths./api/dns/add/...
requestBody:
  content:
    application/json:
      schema:
        $ref: '#/components/schemas/DnsCreate'
    application/x-www-form-urlencoded:
      schema:
        $ref: '#/components/schemas/DnsCreate'
    multipart/form-data:
      schema:
        $ref: '#/components/schemas/DnsCreate'
  required: true

...
components.schemas...:

DnsCreate:
  type: object
  properties:
    name:
      type: string
    dns_type:
      oneOf:
      - $ref: '#/components/schemas/DnsTypeEnum'
      - $ref: '#/components/schemas/BlankEnum'
    content:
      type: string
    ttl:
      type: integer
      maximum: 2147483647
      minimum: -2147483648
      nullable: true
  required:
  - content
  - dns_type
  - name

All 3 content types expect the same content type: DnsCreate. However, this generates this Python code:

def _get_kwargs(
    *,
    body: Union[
        DnsCreate,
        DnsCreate,
        DnsCreate,
    ],
) -> dict[str, Any]:
    headers: dict[str, Any] = {}

    _kwargs: dict[str, Any] = {
        "method": "post",
        "url": "/api/dns/add/",
    }

    if isinstance(body, DnsCreate):
        _kwargs["json"] = body.to_dict()

        headers["Content-Type"] = "application/json"
    if isinstance(body, DnsCreate):
        _kwargs["data"] = body.to_dict()

        headers["Content-Type"] = "application/x-www-form-urlencoded"
    if isinstance(body, DnsCreate):
        _kwargs["files"] = body.to_multipart()

        headers["Content-Type"] = "multipart/form-data"

    _kwargs["headers"] = headers
    return _kwargs

This results in all requests made to this endpoint being treated as multipart form data, and it is impossible to make a JSON or form-urlencoded request.

Desktop (please complete the following information):

• Python Version: 3.12.3
• openapi-python-client version: 0.25.0

Additional context
While it may seem odd to support 3 different content types for the same endpoint, this specification has the advantage of making Django Rest Framework's interactive API support form uploads (instead of requiring you to manually edit JSON). While I'm sure this is a more niche problem, I think we can all agree that this is problematic code generation. While I haven't contributed to this repo, perhaps supporting an additional content type parameter is the solution (rather than relying on isinstance calls).