stellar checkout: x402 / MPP client

[aristidesstaffieri]

Part of the Improve Agent UX for Wallets epic.

Summary

Add stellar checkout <url>, the x402 / MPP client. Standalone and fully net-new — no inherited HTTP or payment-challenge machinery. It performs the payment challenge, drives the on-chain leg through the token / contract invoke pipeline (so the payment inherits the pipeline and is previewable via simulation), then replays proof to fetch the paid resource and returns both the receipt and the service response.

Flow

1. Perform the x402/MPP payment challenge.
2. Drive the on-chain payment leg through the token / contract-invoke pipeline (previewable via --build-only / simulate).
3. Replay proof to fetch the paid resource.
4. Return both the receipt and the service response.

New files

cmd/soroban-cli/src/commands/pay/{mod.rs, ...} — the x402/MPP client (surfaced as the checkout command).

Acceptance criteria

• stellar checkout <url> against an x402 test service drives the on-chain leg through the token pipeline and returns both the receipt and the fetched response.

Dependencies

Depends on the stellar token command (reuses its pipeline for the on-chain leg) and the structured-output/CLI-API task.