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)

No specific files or tests are listed, so first find the realtime agent session-init path where tool schemas are set and the meta-tool flow that mutates tool availability at runtime. Review any in-repo docs for realtime API limits to confirm schema mutability assumptions. Then inspect related examples/tests for realtime agents to validate expected behavior. Completion is a documented feasibility conclusion (and scope delta) covering options like pre-expanding tools or restarting on tool change.

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