feat(contracts/market): verify withdraw cooldown respects LastDepositTime (#565)

[davismas23]

Summary

Closes #565

Documents and adds targeted unit tests confirming the withdrawal cooldown behaviour introduced in issue #413.

Problem

The WITHDRAW_COOLDOWN_SECONDS (3 600 s / 1 hour) guard in withdraw_unused_collateral was reading LastDepositTime correctly, but:

• There were no dedicated unit tests proving the early-withdraw rejection and the post-cooldown success paths.
• The module documentation had no explanation of the cooldown contract, leaving the deposit → timestamp → cooldown relationship implicit.

Without explicit test coverage, a future refactor could accidentally drop the check and CI would not catch it.

Changes

contracts/market/src/withdraw.rs

Documentation — new ## Withdrawal cooldown (#413 / #565) section in the module-level doc comment:

• Explains the guard purpose (deter deposit/withdraw round-trips)
• Shows the full data-flow: deposit_collateral → set_last_deposit_time → withdraw check
• Documents the no-deposit-record edge case

Tests — three new unit tests in the existing #[cfg(test)] mod tests block:

Test	What it verifies
test_withdraw_blocked_within_cooldown	Returns WithdrawCooldownActive when elapsed = 0 < 3 600
test_withdraw_allowed_after_cooldown	Succeeds when elapsed = 3 600 >= 3 600 (exact boundary)
test_withdraw_no_deposit_record_bypasses_cooldown	No LastDepositTime → cooldown is not triggered

Core implementation (unchanged)

// 3. Enforce cooldown: user must wait WITHDRAW_COOLDOWN_SECONDS after their last deposit.
if let Some(last_deposit_time) = storage::get_last_deposit_time(&env, market_id, &user) {
    let elapsed = env.ledger().timestamp().saturating_sub(last_deposit_time);
    if elapsed < WITHDRAW_COOLDOWN_SECONDS {
        return Err(ContractError::WithdrawCooldownActive);
    }
}

Acceptance criteria

• Early withdraw (0 s elapsed) fails with WithdrawCooldownActive
• Withdrawal at exactly the boundary (3 600 s elapsed) succeeds
• User with no deposit record is not blocked by cooldown

Notes

• PR is focused exclusively on issue Withdraw cooldown respects LastDepositTime #565
• No storage schema changes
• No behaviour changes — tests only confirm existing logic