test(33_Airdrop): cover allowance boundary cases

[wuyua9]

What

Add a focused Foundry regression test for 33_Airdrop/Airdrop.sol covering the multiTransferToken boundary behavior.

Why

The contract must allow an allowance exactly equal to the sum of the airdrop amounts. The test also preserves the failure behavior for insufficient allowance and mismatched recipient/amount arrays, so a future change from >= back to > is caught.

Scope

• Add test/Airdrop.t.sol only.
• Cover exact allowance success and recipient balance/allowance results.
• Cover insufficient allowance and mismatched array reverts.
• No contract or documentation changes.

Validation

• git diff --check passed.
• Static test-shape checks passed.
• Local forge test --match-path test/Airdrop.t.sol -vvv could not run because forge is not installed in the submission environment. Please rely on CI for Foundry compilation and execution.