feat(realtime): support meta tool in realtime agent

**Background**

Realtime APIs require tool schemas to be submitted at session initialization and cannot be modified mid-session. This conflicts with AgentScope's meta tool mechanism, which dynamically expands or switches tools at runtime. It is unclear whether meta tools can be supported in realtime agent at all.

We need to first investigate and document the constraint imposed by realtime API on tool schema immutability
Evaluate feasibility of meta tool support (e.g., pre-expanding all possible tools at session start, or session restart on tool change)