ThresholdSigningMultisig tests

[derekpierre]

Type of PR:

• Bugfix
• Feature
• Documentation
• Other

Required reviews:

• 1
• 2
• 3

What this does:

Add tests for threshold multisig.

Issues fixed/closed:

• Fixes #...

Why it's needed:

Explain how this PR fits in the greater context of the NuCypher Network.
E.g., if this PR address a nucypher/productdev issue, let reviewers know!

Notes for reviewers:

What should reviewers focus on?
Is there a particular commit/function/section of your PR that requires more attention from reviewers?

[Copilot]

Pull Request Overview

This PR adds comprehensive test coverage for the ThresholdSigningMultisig contract, which implements a multi-signature wallet with ERC-1271 signature validation.

Key changes:

• Adds 10 test functions covering initialization, signer management, threshold configuration, authorization checks, and signature validation
• Tests both individual operations (add/remove/replace signers) and bulk updates with and without full signer replacement
• Validates signature verification logic including edge cases and invalid scenarios

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[vzotova]

looks great