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.

从 WorkItemAttachments.create() 和 WorkItemAttachment 模型入手，然后将它们的验证路径与文档中说明的 Plane wrapper 响应进行比较。使用 WorkItemAttachmentUploadRequest 重现该调用，并验证结果在没有 ValidationError 的情况下同时暴露嵌套的 attachment 记录和 upload_data。