Fix x402 payment data leak: settle before handler execution

[shafu0x]

Summary

• Moves x402 settlement before handler invocation in the orchestration flow
• Previously: verify → execute handler → settle. This allowed concurrent requests with the same payment nonce to both pass verification, both get data, but only one actually settles on-chain. The second request gets free data.
• Now: verify → settle → execute handler. If settlement fails (duplicate nonce, insufficient funds), the handler never runs and no data is returned.
• Changes settlement failure status from 500 to 402 (the payment didn't go through, so it's a payment error)
• Updates tests to reflect the new correct behavior: settlement happens regardless of handler outcome

Fixes Merit-Systems/enrichx402#97