fix: WorkItemAttachments.create() raises ValidationError on actual Plane response

Calling client.work_items.attachments.create(...) against a live Plane instance raises:

pydantic.ValidationError: 1 validation error for WorkItemAttachment
asset
  Field required [type=missing, input_value={'upload_data': {...}, 'asset_id': '...', 'attachment': {...}, 'asset_url': '...'}, input_type=dict]

Plane returns a wrapper response, not a flat WorkItemAttachment:

{
  "upload_data": { "url": "...", "fields": { "...S3 multipart policy..." } },
  "asset_id": "427e...",
  "attachment": { "id": "427e...", "asset": "ws/uuid-name.ext", "is_uploaded": false, ... },
  "asset_url": "/api/assets/v2/.../attachments/427e.../"
}

But WorkItemAttachments.create() calls WorkItemAttachment.model_validate(response) directly on the wrapper — WorkItemAttachment is actually nested inside response["attachment"]. Hence the missing-asset validation error.

Reproduction:

from plane import PlaneClient
from plane.models.work_items import WorkItemAttachmentUploadRequest

client = PlaneClient(api_key="<key>", base_url="https://your-plane.example.io")
client.work_items.attachments.create(
    workspace_slug="ws", project_id="<uuid>", work_item_id="<uuid>",
    data=WorkItemAttachmentUploadRequest(name="x.txt", size=10),
)
# raises pydantic.ValidationError

Suggested fix: add a new WorkItemAttachmentCreateResponse model capturing the full wrapper and change WorkItemAttachments.create() to return it. Exposes both the attachment record and the upload data the caller needs for the S3 multipart
POST.

PR coming.

Comienza por WorkItemAttachments.create() y el modelo WorkItemAttachment; después compara su ruta de validación con la respuesta documentada del wrapper de Plane. Reproduce la llamada usando WorkItemAttachmentUploadRequest y verifica que el resultado exponga tanto el registro de attachment anidado como upload_data sin un ValidationError.