Add deposit_revenue payment-token locking tests for PaymentTokenMismatch enforcement

[thlpkee20-wq]

Description

The README states the payment token is locked at the first successful deposit and all later deposits must match, returning PaymentTokenMismatch otherwise, with get_payment_token returning None before the first deposit. These invariants need direct tests: first-deposit locking, mismatch rejection, and pre-deposit None. Add a focused suite.

Requirements and context

• Must be secure, tested, and documented
• Should be efficient and easy to review
• Relevant code: src/lib.rs (deposit_revenue, get_payment_token), src/payout_test.rs
• Use try_* clients to assert PaymentTokenMismatch

Suggested execution

• Fork the repo and create a branch
• git checkout -b feat/payment-token-locking-tests
• Implement changes
  • Assert get_payment_token is None before any deposit
  • Deposit with token A, then assert deposit with token B fails with PaymentTokenMismatch
  • Assert get_payment_token returns A after the first successful deposit
• Validate security and correctness assumptions

Test and commit

• Run tests
  • cargo test
• Cover edge cases
  • Deposit on unknown offering, repeated same-token deposits, mismatch on first override
• Include test output and security notes

Example commit message

test: cover payment token locking and mismatch enforcement

Guidelines

• Minimum 95 percent test coverage
• Clear documentation
• Timeframe: 96 hours