feat: Add haystack quiz agent — multi-source quiz generator

[thienphuquach]

Summary

This adds a quiz agent to haystack-agents/ that I built for the Fetch.ai Innovation Lab internship. The core idea: give it any URL or PDF, pay once via Stripe, and get a grounded multiple-choice quiz delivered entirely through ASI:One Interactive Cards , no external app, no file downloads.

A few things worth noting about the implementation:

• Payment is verified server-side before any quiz logic runs. The Stripe session ID is checked against the Stripe API on CommitPayment, not just trusted from the client message.
• Wikipedia had to be handled separately direct scraping returns 403, so Wikipedia URLs are routed through the MediaWiki REST API which gives back clean plain text.
• PDFs from ASI:One arrive as Cloudinary markdown (![file.pdf](url) inside a TextContent block), not as a ResourceContent.
• No OpenAI embedding key needed — retrieval (Retry Weak Topics, Study Concept) uses BM25 only.
• brotlicffi is pinned in requirements.txt — Python 3.13 broke the old brotli package; this is the compatible replacement for both aiohttp (mailbox) and requests (URL fetching).

Demo video: https://youtu.be/RmrOGutIjbg

Type of Change

• New agent example

Checklist

• I have starred this repository.
• New community agents are under contributors/<agent-name>/. (This is an Innovation Lab example under haystack-agents/, not a community contribution.)
• I ran ruff check ..
• I ran ruff format ..
• I added/updated README.md.
• I added .env.example.
• I added demo image/GIF. (Architecture diagram + YouTube walkthrough linked in README.)
• I added agent profile link.
• I updated CHANGELOG.md.
• I added my agent to the Community Contributors table in root README.md.
• I verified paths/commands in docs.
• I understand this PR requires maintainer review before merge.

Notes for Reviewers

.env is gitignored — no secrets in the diff. The only sensitive-looking file is .env.example, which contains only placeholder values and comments.