Add AI-generated content declaration option

[ishowshao]

Summary

• Add a new xhs post --ai-generated flag.
• Pass the flag through to create_image_note.
• Include userDeclarationBind: {"origin": 2} in business_binds when enabled.
• Add a unit test for the generated publish payload.

Context

The creator web publish flow marks notes containing AI-generated content through the /web_api/sns/v2/note payload by setting business_binds.userDeclarationBind.origin to 2. This exposes that behavior as an explicit CLI option while keeping the default publish payload unchanged.

Test plan

• uv run ruff check xhs_cli/commands/creator.py xhs_cli/client_mixins.py tests/test_client.py
• uv run pytest tests/test_client.py::TestCreatorEndpoints tests/test_creator.py