Add FromMultipart[T] for complex structured multipart form data

[Copilot]

Extends the initial FromFiles/FromText implementation to support complex structured data in multipart forms. Users can now bind dataclasses with mixed regular fields, nested objects, and file uploads (bytes fields) with full type safety and automatic OpenAPI documentation.

Changes

• Added FromMultipart[T] and MultipartBinder: New binder for complex multipart types with automatic form-to-object binding
• OpenAPI schema generation: Introspects bound types and generates proper schemas where bytes fields → type: string, format: binary
• Refactored _get_binder_by_type(): Extracted common pattern from _get_body_binder() and _get_files_binder() to reduce duplication

Example

from dataclasses import dataclass
from uuid import UUID
from blacksheep import Application, FromMultipart

@dataclass
class Address:
    street: str
    city: str

@dataclass
class UserProfile:
    id: UUID
    addresses: list[Address]
    avatar: bytes  # File upload

app = Application()

@app.router.post("/profile")
async def create_profile(data: FromMultipart[UserProfile]):
    # data.value.id is UUID
    # data.value.addresses is list[Address]
    # data.value.avatar is bytes from file upload
    return {"status": "created"}

Generated OpenAPI spec:

requestBody:
  content:
    multipart/form-data:
      schema:
        type: object
        properties:
          id:
            type: string
            format: uuid
          addresses:
            type: array
            items:
              $ref: '#/components/schemas/Address'
          avatar:
            type: string
            format: binary

Original prompt

Please continue the PR, working on the last comment I just wrote #652

Created from Copilot CLI via the copilot delegate command.

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