`abort_signal` (asyncio.Event) is passed via **kwargs into payloads during
tool_call streaming, causing "Object of type Event is not JSON serializable"
when the OpenAI client tries to serialize the request body.
Regression test added: test_prepare_chat_payload_strips_non_json_serializable_kwargs
The dev branch has astrbot/dashboard/dist as a symlink to
../../dashboard/dist, which is valid in the dev workspace but
becomes a broken symlink when cloned to /opt/astrbot for installation.
Fix the maturin build hook to:
- Remove broken symlinks before creating placeholder directories
- Handle symlink vs directory removal in copy_dashboard_dist()
- Always generate placeholder when dashboard build is skipped or fails