feat: session summary via agent.get_summary() for cascading & realtime

[DeepBhupatkar]

What

Adds a new get_summary method to Agent, allowing users to retrieve a concise session summary (for example, at the end of a call).

This works for both Cascading and Realtime pipelines.

Usage

Cascading Pipeline (can be use pipeline LLM)

summary = await agent.get_summary()

Realtime Pipeline (requires text LLM)

llm = OpenAILLM(model="gpt-4o")
summary = await agent.get_summary(llm=llm)