[huangji6693-max]

Closes #568

Problem

Neither escrow::contribute nor milestones::contribute checked the stored deadline. Once now >= deadline + GRACE_PERIOD, escrow::refund and milestones::cancel_milestone_after_deadline become permissionless, yet both contracts continued accepting new contributions that could be immediately refunded or cancelled by any third party in the next transaction.

Solution

• Reject escrow::contribute with Error::InvalidDeadline once env.ledger().timestamp() >= escrow.deadline.
• Reject milestones::contribute with Error::DeadlinePassed once env.ledger().timestamp() >= milestone.deadline (matching milestones::allocate).
• Add regression unit tests (test_contribute_rejects_at_or_after_deadline) in both mergefi-escrow and mergefi-milestones verifying now == deadline - 1 (succeeds), now == deadline (rejected), and now == deadline + GRACE_PERIOD (rejected).

Verification

• CARGO_BUILD_JOBS=2 cargo test -p mergefi-escrow (99/99 passed)
• CARGO_BUILD_JOBS=2 cargo test -p mergefi-milestones -- test_contribute_rejects_at_or_after_deadline (passed)
• CARGO_BUILD_JOBS=2 cargo clippy --workspace --lib --locked -- -D warnings (0 warnings)

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