[codex] Cover persisted MPP order identifiers

[Dreamstore2046]

Summary

• add transaction-log coverage for the MPP order-id contract
• make the payForMedication test fs mock persist appended JSONL transaction records
• assert a successful MPP payment with no real Stellar hash persists mppOrderId and does not persist stellarTxHash

Closes #20

Validation

• npm test -- agent/__tests__/pay-for-medication.test.ts
• git diff --cached --check

Notes

• The production path already stores mppOrderId separately and normalizes invalid/non-hex receipt values to stellarTxHash: undefined; this PR adds the missing persistence-level regression coverage requested by the issue.
• A broader run including agent/__tests__/mock-network.test.ts is still blocked in this Windows checkout by an existing path/mock issue (mkdir 'C:\C:\Users\...'), unrelated to this PR.
• A focused TypeScript command that imports agent/tools.ts is still blocked by existing x402 template literal type errors in agent/tools.ts:264 and agent/tools.ts:266.

[vercel]

@Dreamstore2046 is attempting to deploy a commit to the Harrison Eze's projects Team on Vercel.

A member of the Team first needs to authorize it.