feat: Add .get_turns(tool_result_role=...); fix/improve .app() method

[cpsievert]

Features/improvements inspired by the following problem

import chatlas as ctl

chat = ctl.ChatAnthropic()

def get_current_temperature(latitude: float, longitude: float):
    return "72°F."


chat.register_tool(get_current_temperature)

chat.chat("How is the weather in Duluth, MN today?")

# Doesn't render the tool displays correctly
chat.app()