docs(refunds): clarify conditional IBAN/BIC requirement

[jfrosorio]

Closes #76.

Live testing on the sandbox (2026-07-17) confirmed that the refund iban and bic fields, while marked optional in Eupago's API docs, are conditionally required: methods without a direct refund path (e.g. Multibanco) reject with IBAN_MISSING unless an IBAN is given, and an IBAN without its matching BIC is rejected with BIC_INVALID — Eupago does not derive the BIC. Direct-path methods such as MB WAY need neither.

This updates docs/refunds.md to spell that out, including the rejection codes users will see.

[Copilot]

Pull request overview

Updates the refunds documentation to reflect Eupago’s observed “conditionally required” behavior for iban/bic during refunds, reducing failed refund attempts caused by relying on the upstream API docs’ “optional” labeling.

Changes:

• Clarifies that direct-path refund methods (e.g. MB WAY) require no iban/bic.
• Documents that non-direct-path methods (e.g. Multibanco) require an iban, otherwise Eupago rejects with IBAN_MISSING.
• Documents that when iban is provided, the matching bic must also be provided (no derivation), otherwise Eupago rejects with BIC_INVALID.

---

💡 Add Copilot custom instructions for smarter, more guided reviews. Learn how to get started.