Support array of format: binary items in multipart/form-data request bodies

Describe the bug
When an OpenAPI 3.0 schema defines a property as an array of binaries inside a multipart/form-data requestBody, the generator currently treats it as a JSON body and emits a single body parameter. As a result:

• The generated sync_detailed(...) signature has only:

def sync_detailed(
    *,
    client: Client,
    body: UploadFilesPostBody,
    hash_: str,
) -> Response[Any]:
    …

• Internally body.files (a list of File objects with payload: BinaryIO) is passed to json=…, causing serialization errors.

OpenAPI Spec File

paths:
  /upload_files:
    post:
      requestBody:
        content:
          multipart/form-data:
            schema:
              type: object
              properties:
                hash_:
                  type: string
                files:
                  type: array
                  items:
                    type: string
                    format: binary
      responses:
        '200':
          description: OK

What actually happens

• The generator emits a single body: UploadFilesPostBody parameter.
• When calling sync_detailed(..., body=…), httpx is invoked with json=… and fails on BinaryIO because it isn’t valid JSON.

Desktop:

• OS: Windows 10
• Python Version: cPython 3.12.8
• openapi-python-client version 0.24.3

Additional context
None - if anything is needed, please let me know

Start by tracing the generator path that handles multipart/form-data request bodies and compare it with the generated sync_detailed(...) method described in the issue. Verify the generated client sends the files field as multipart data rather than through httpx json=, and add coverage for the supplied OpenAPI schema if the repository has a relevant generator test location.