fix: authorize flash loan repayment (#315) and restore compilation

[laurentketterle-hub]

Summary

Two fixes in the investment_vault crate:

1. Flash loan repayment authorization (#315)

execute_flash_loan settled the repayment with Base::transfer(&env, &borrower, &vault, amount + fee).
Base::transfer calls borrower.require_auth(), which a contract borrower cannot satisfy once its
flash_loan_callback has already returned — the authorization window is tied to the callback invocation,
so the repayment only worked under mock_all_auths() and would fail on a real network.

The fix moves the shares directly with the unauthenticated Base::update primitive (the same primitive
Base::transfer/Base::burn use internally, minus require_auth). The borrower's consent is already
established by returning true from flash_loan_callback in the same transaction, and the entry point is
gated by initiator.require_auth().

2. Restore compilation (broken on main)

main currently does not compile (cargo check -p investment-vault fails). This PR restores the build:

• Add missing closing braces to get_withdrawal_window, get_volume_fee_tier and check_deposit_lock in lib.rs.
• Add missing closing braces to withdrawal_window_set and funding_round_ended in events.rs.
• Add missing closing brace to test_volume_fee_tier_is_admin_only in test.rs.
• Fix duplicate VaultError discriminants (FundingRoundActive = 42, InvestmentCapExceeded = 43; all three were 41).

Testing

cargo check -p investment-vault passes (the lib compiles). The full cargo test suite requires a fresh
stellar contract build (the wasm artifacts are gitignored and rebuilt by CI), so it could not be
exercised in the local environment.

Closes #315