Harden x402 paid-action example responses

[TateLyman]

Summary

• mark x402 paid-action example JSON responses as Cache-Control: no-store
• assert non-cacheable headers on successful/idempotent receipts and payment-error responses
• sync package-lock.json with package.json so clean npm ci installs include @neondatabase/serverless

Why

The example links payment acceptance, agent execution, and emitted receipts. Those responses can include payment/action correlation metadata, so they should not be cached by browsers, proxies, or local tooling.

Verification

• npm ci
• node --test tests/x402-paid-action-receipt.test.js
• git diff --check

Note: I also tried the full npm test suite after npm ci; it progressed past the SIWE dependency path, then hung on the ENS/provider-owned-name test path, so I stopped it and kept verification scoped to the files changed.

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