feat(agent): support multiple tool calls

[ehhuang]

🚀 Describe the new functionality needed

A lot of checks currently on assert(len(tool_calls) == 1). We should support multiple tool calls.

💡 Why is this needed? What if we don't build it?

Table stacks for any agent framework.

Other thoughts

No response

[terrytangyuan]

#1120 might be a prerequisite if we want to use remote vLLM + agent.