docs(x402): put the API key before the code example and end "check that it works" with a paid Sepolia settlement #2413

What happens

On build/agents/x402.mdx (last changed by #2397, merged 2026-10-21) the order is:

1. "Pointing a Resource Server at It" — code example reading X402_API_KEY
2. "Getting an API Key"
3. "Check that it works" — an unpaid curl that returns 402 and, as the page says, "needs no API key"

A reader who copies the example and runs the check gets a green result without ever creating a key. The facilitator's /verify and /supported are open and only /settle needs the key, so the integration looks healthy until a real buyer pays.

This is the path one seller took: 8 public hosts advertising 1,062 Celo routes, 15 of 15 paid attempts by an independent buyer refused at /settle with 401 Missing X-API-Key, zero USDC moved (vet402 public ledger, 2026-10-19 → 10-26). The page already states the fact ("an integration can look healthy until its first settlement"); the order defeats it.

Suggested fix

• Move "Getting an API Key" above the code example and make X402_API_KEY a stated prerequisite.
• End "Check that it works" with a paid step on Celo Sepolia through the key (the Celo Composer template's pnpm api:buy already does this), so the check fails without a key.
• One sentence at the first mention of /verify: verifying succeeds without a key; settling does not.

The same reorder is a box on the facilitator repo's docs/RESOURCE_SERVER.md (celo-org/x402-facilitator#369).

Acceptance criteria

• Key section precedes the code example
• "Check that it works" ends with a paid Sepolia settlement and names the 401 a keyless seller would see
• mint validate and mint broken-links pass

Refs #2263, #2397.