agent: add adapter for Leboncoin

[alectimison-maker]

Summary

• add a narrowly scoped Leboncoin adapter for French consumer classifieds pages
• mirror the guidance across Chrome and Firefox
• stop safely on the observed HTTP 403/blank response without inferring no listings
• distinguish browsing, messaging/offers, secure delivery, and in-person handoff/payment
• require platform payment confirmation before an in-person seller hands over the item
• keep payment, tracking, receipt, problem evidence, wallet, and bank-settlement states separate
• reject pro/help, embedded-domain, and suffix-look-alike URLs
• add positive, negative, content, bullet-count, and browser-parity coverage

Research

The guidance is dated 2026-08 and reflects the current consumer access response plus Leboncoin's official assistance articles for Transaction sécurisée, delivery, in-person handoff, payment protection, wallet settlement, and Adyen verification.

Test plan

• node test/run.js — new Leboncoin coverage passes; 1412 passed / 1 inherited failure because package.json is 26.0.4 while the newest CHANGELOG.md entry is 26.0.0
• npm run test:fixtures — 125/125 passed
• npm run test:security — 60/60 passed
• npm run test:ci — passed
• npm run test:webmcp — passed with Chrome 150.0.7871.187

Anonymous LLM scenarios were not run because they require an externally configured provider/API key and interactive headed setup; they are not part of the adapter-specific test seam.