fix(refund): suspend deadline-based refunds while invoices are under dispute

[johnsaviour56-ship-it]

Summary

This PR prevents automatic refunds from being processed while an invoice is actively under dispute, ensuring that arbitration takes precedence over deadline-based refund logic.

Changes

• Updated refund() to check the invoice's dispute status before evaluating deadline eligibility.
• Blocked refunds for invoices marked as disputed, returning an "invoice under dispute" error instead of processing the refund.
• Preserved existing deadline-based refund behavior for invoices that are not under dispute.
• Allowed refunds to proceed immediately after the dispute is resolved and the disputed flag is cleared.
• Added tests covering disputed and non-disputed refund scenarios.

Testing

Added coverage for:

• Refund attempts on disputed invoices after the deadline
• Successful refunds once a dispute has been resolved
• Unchanged behavior for non-disputed invoices
• Regression tests for existing refund logic

Acceptance Criteria

• refund() rejects disputed invoices, even after the deadline has passed
• Refund eligibility resumes immediately after dispute resolution
• Non-disputed invoice behavior remains unchanged
• Added test covering dispute resolution and subsequent refund
• All existing tests pass
• cargo clippy passes with zero warnings

Result

Deadline-based refunds are now safely suspended during active disputes, preventing premature fund distribution while arbitration is in progress and preserving the expected refund workflow once disputes are resolved.

Closes #225

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