docs: add Agent402 to the sample MPP endpoints (Tempo)

[MikeyPetrillo]

Adds Agent402 to the sample MPP endpoints table in Tutorial 08 (08-mpp-machine-payments-protocol).

Agent402 (agent402.tools) is an open-source catalog of 500+ deterministic tools; every paid endpoint serves an MPP tempo/charge challenge in USDC.e on Tempo mainnet (chain 4217) alongside x402 on the same 402. The listed endpoint returns sha256("hello world") for the example body, so a buyer can verify the paid response offline - a cheap ($0.001), repeatable happy-path target.

Verified live with the AgentCore Payments plugin (Stripe Privy instrument) buying it end to end on Tempo mainnet.

To verify the row: the endpoint returns HTTP 402 with a WWW-Authenticate: Payment tempo/charge challenge -

curl -i -X POST https://agent402.tools/api/hash \
  -H 'content-type: application/json' \
  -d '{"text":"hello world","algo":"sha256"}'