Add integration test for the investor list endpoint returning all commitments for a funded invoice

[Chucks1093]

Summary

The investor list endpoint on a funded invoice should return every investor commitment with their wallet (truncated), amount, share percentage, and commitment timestamp. An integration test should seed multiple commitments and verify all are present and correctly shaped.

Scope

• Seed a funded invoice with three investor commitments of different amounts
• Call the investor list endpoint for that invoice
• Assert all three investors are returned with correct amount and share percentage
• Assert share percentages sum to 100%
• Assert a different seller cannot access the investor list for an invoice they do not own (expect 403)

Acceptance Criteria

• All three investors present in the response
• Share percentages sum to 100%
• Each entry includes wallet, amount, share_percent, and committed_at
• 403 returned when a different seller calls the endpoint

ETA: 12 hours

---

Coordinate on Telegram