[WIP] Add test coverage for ICMP module

[Copilot]

Thanks for assigning this issue to me. I'm starting to work on it and will keep this PR's description up to date as I form a plan and make progress.

Original prompt

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This section details on the original issue you should resolve

<issue_title>test: ICMP</issue_title>
<issue_description>## Problem

Magma.Internet.Icmp exists in src/ but has zero test coverage.

Sub-tasks

• Create test/Magma.Internet.Icmp.Facts project, add to solution
• Echo request construction and field validation
• Echo reply parsing
• Checksum computation against known packets
• Type/code enumeration coverage

Dependencies

Depends on: #99 (CI to run them)
Unlocks: #110 (ICMP improvements + ICMPv6)

Phase

Phase 1 (Core Protocol & Tooling)</issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes test: ICMP #109

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