Proposal: Add integrations/agent402/ sample - x402 buy + sell side

**Proposal: Add `integrations/agent402/` sample - x402 buy + sell side**

Agent402 is an open-source x402 + MCP server with a catalog of 500+
pay-per-call endpoints (400+ tools + 100+ skill packs), plus
`agent402-tollbooth` for pay-per-crawl on the other side.
Both speak vanilla x402, so an AgentCore-hosted Strands agent works
end-to-end with no protocol bridging.

Happy to PR a small `integrations/agent402/` folder containing:
1. A buy-side Strands agent calling Agent402 tools via the published
`agent402-strands` adapter (proof-of-work free tier; AgentCore Payments
+ CDP signs for wallet-only tools). Working code:
https://github.com/MikeyPetrillo/Agent402/tree/main/examples/agentcore
2. A sell-side reverse-flow demo: a Strands agent paying a
self-hostable tollbooth gate. Working code:
https://github.com/MikeyPetrillo/Agent402/tree/main/examples/agentcore-tollbooth
3. A README pointing at the 5-minute integration guide:
https://github.com/MikeyPetrillo/Agent402/wiki/AWS-Bedrock-AgentCore

Closes the loop for the AgentCore Payments demo
(`aws-samples/sample-agentcore-cloudfront-x402-payments`) by giving it a
real buy-side counterparty + an open-source sell-side gate.

Will follow this issue with a PR once you confirm the folder location.

Maintainer: Havok Holdings LLC

Start by reviewing the proposed integrations/agent402/ location and the linked AgentCore examples for the buy-side and agentcore-tollbooth flows. Use the published agent402-strands adapter and the five-minute integration guide as references. Done means the sample includes both x402 flows and a README linking the guide.

Written by the indexing model from the issue text.