Add AP2 detection and ERC-8004 service link #5
Description
Extend A2A agent card validation with:
- AP2 (Agent Payments Protocol) detection from A2A-Extensions header and agent card extensions
- ERC-8004 service link detection from agent card services array
- ap2Version, erc8004ServiceUrl, extensions entries
2 new categories, 3 new entries, ~18 new tests.