[Feature]: fork session

[gusye1234]

Feature Description

Support fork a session (copy the current session to a new session id)

Proposed API/Interface

# Python SDK
# Your proposed API here
r = client.sessions.fork(session_id=....)
print(r.new_session_id, r.old_session_id)

Impact Component/Area

Which part of Acontext would this feature affect?

• Client SDK (Python)
• Client SDK (TypeScript)
• Core Service (Python)
• API Server (Go)
• UI/Dashboard (Next.js)
• CLI Tool
• Documentation
• Other (please specify)