Fix Reentrancy Vulnerability in `executeTransaction`

[seriousfuzzy]

hello @hackfisher @yrong @hujw77 @WoeOm @xiaoch05 ,
please check this PR.

Changes:

• Updated executeTransaction to set executed = true before external calls.
• Used CEI pattern to prevent reentrancy.
• Added fallback to send for safe Ether transfers.
• Improved error handling on external calls.

Testing:

• Added unit tests for reentrancy scenarios.
• Verified successful and unsuccessful transaction execution behavior.

[hujw77]

Thank you so much for your contribution!

1. This contract is a fork of the Gnosis MultiSigWallet, and you can also submit this PR to them.
2. We’ve already switched to using the Gnosis Safe Wallet in the production environment.
3. The original code already includes reentrancy checks.
4. This is an older version of the contract, so your modifications may not compile and will need to be adapted for the older solidity compiler.

Given these reasons, we won’t be merging the PR for now.

[seriousfuzzy]

Thank you.