Fixed schema URLs - missing v2

[jlwasmer]

Replaced
https://adcontextprotocol.org/schemas/adagents.json with
https://adcontextprotocol.org/schemas/v2/adagents.json

[bokelley]

Closing in favor of #335, which takes an alternative approach: instead of hard-coding v2 URLs, it removes the default and example $schema references entirely since they weren't providing functional value and were the source of the version drift issue.