Least Authority audit follow-up: require XMR quote refund addresses and validate all supplied refund addresses

Context

This MR follows up on Suggestion 3 from Least Authority’s XMR audit review:

Make refund_address required for requests to the quote endpoints.

Least Authority noted that both quote endpoints could return parsable memos without a refund address. That is unsafe for source chains such as XMR, where the sender cannot be reliably derived from the inbound transaction and therefore cannot serve as a fallback refund destination.

This implementation scopes the requirement to chains that require an explicit refund address (currently XMR) rather than making the field globally mandatory. When refund_address is supplied for any chain, it is validated against the source chain and configured network. XMR addresses must also use a supported address form.

Changes

• Require refund_address for both swap and limit quotes when the source chain requires an explicit refund address.
• Resolve THORName aliases before validation.
• Validate supplied refund addresses against:
  • the actual source chain,
  • the configured network,
  • supported XMR address forms.
• Preserve optional refund addresses for ordinary source chains and THOR-sourced assets such as XMR synths.
• Preserve valid legacy UTXO address formats.
• Include the resolved refund address in generated swap and limit-order memos.
• Update protobuf and OpenAPI documentation and generated artifacts.
• Repair XMR quote memo regression fixtures to use source-chain BTC refund addresses.

Test coverage

Covers both swap and limit quotes for:

• missing XMR refund addresses,
• wrong-chain and wrong-network addresses,
• unsupported XMR address forms,
• valid XMR standard addresses and subaddresses,
• THORName resolution,
• malformed addresses,
• BTC bech32 and legacy addresses,
• optional ordinary-chain refunds,
• XMR synth source-chain behavior,
• generated memo contents.