fix(protocol): std leaking into tests

[refcell]

Description

Fixes the maili-protocol crate to not have std leaking into tests.

Closes #224

[refcell]

📚 $\text{Stack Overview}$

Pulls submitted in this stack:

• fix(protocol): std leaking into tests #241 👈
• main

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 95.4%. Comparing base (64066b7) to head (894345c).
Report is 1 commits behind head on main.

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