fix(payments): require password for all sends [AMB-3265]

[bufo24]

Summary

• require a non-empty password for every SDK send, including sandbox
• allow any non-empty sandbox placeholder such as Password123 because sandbox does not use the value
• require the real team password for live sends
• reject missing or blank passwords before wallet lookup or transaction creation
• keep prepared credentials scoped to retryPayment and update SDK documentation/examples

Testing

• 61 payments tests passed for the original change
• 13 focused send/preparation tests passed after the placeholder clarification
• payments TypeScript typecheck passed
• ESM and CommonJS builds passed
• CommonJS examples typecheck passed
• formatting checks passed

Linear: https://linear.app/amboss/issue/AMB-3265/require-team-password-for-sandbox-and-production-payments-sdk-sends