Issue 318 flash loan repayment validation

[auracule007]

Summary #318

Implements flash loan repayment validation for the flash_loan_vault contract.

The vault now explicitly checks that, after the receiver callback completes, its token balance is at least the original pre-loan balance plus the required flash loan fee. If the callback fails to repay the borrowed principal plus fee, the contract returns Error::LoanNotRepaid.

Changes

• Added an explicit required_balance = pre_balance + fee repayment check
• Return Error::LoanNotRepaid when repayment is insufficient
• Clarified contract docs around repayment expectations
• Added test coverage for:
  • Receiver that does not repay the loan
  • Receiver that repays principal only but omits the required fee
• Verified failed repayment does not leave the vault drained or inflate deposited totals

Validation

• cargo fmt --all -- --check
• cargo clippy --locked --all-targets --all-features -- -D warnings
• cargo test --locked

Notes

The full test run generated unrelated snapshot updates as a side effect; those were reverted. The PR only modifies the flash loan vault contract and tests.

Closes #318

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[auracule007]

@EDOHWARES Please review this PR and check workflow