refactor: Add RunTestPaymentService and self-pay checks

[ValeraFinebits]

This PR stabilizes the RunTestPayment([FromBody] RunTestPaymentRequest, CancellationToken) self-pay diagnostic flow and makes failures easier to understand.

RunTestPayment([FromBody] RunTestPaymentRequest, CancellationToken) is a cheat-mode-only scenario where sender and receiver can operate against the same store wallet. That makes it useful for finding same-wallet payjoin edge cases, but it also exposed a flaky dust-output failure caused by sender/receiver change handling colliding in this self-pay path.

Changes

• fixed the self-pay dust issue by reserving the sender change address before building the payjoin proposal;
• extracted the self-pay workflow into RunTestPaymentService;
• extracted same-wallet diagnostic checks into SelfPayInvariantChecker;
• improved invariant logging by splitting warning summary from full debug diagnostics;
• added targeted unit coverage and kept the self-pay integration flow passing.

added explicit self-pay invariant validation for:

• invoice output preservation,
• receiver contribution,
• sender change script reuse,
• zero-value outputs,
• dust outputs;

Why

This keeps RunTestPayment([FromBody] RunTestPaymentRequest, CancellationToken) useful as a deliberately weird same-wallet diagnostic scenario, while making it less flaky and much more actionable when it fails.