support fragments plugin for Python api

[kj-9]

seems like fragments plugin is not work for python api at current llm version.

with

• "llm==0.24.2"
• "llm-hacker-news==0.1"

import llm
model = llm.get_model()
prompt = "Summarize this Hacker News thread"
response = model.prompt(prompt, fragments=['hn:43632049'])

response is asking me to provide actual hacker news comments, not comments' id.