Normalize array initialization in DotnetTest IPC serializers

[Evangelink]

Fixes #9786.

Summary

Follow-up to #9774. PR #9774 introduced two different empty-array initialization styles across the DotnetTest IPC serializers. This normalizes the three inconsistent files to the = [] pattern used by DiscoveredTestMessagesSerializer and TestInProgressMessagesSerializer, removing the nullable = null initializer and the ?? [] null-coalescing at the return site.

Files changed

• CommandLineOptionMessagesSerializer.cs
• FileArtifactMessagesSerializer.cs
• TestResultMessagesSerializer.cs

Notes

The missing InternalAPI.Unshipped.txt declarations (ReadFields/WriteListPayload and PlatformServicesConfigurationAdapter) referenced in the issue are already present on main, so no InternalAPI changes were needed.

No functional changes — behavior is identical.

Testing

• ✅ Microsoft.Testing.Platform builds cleanly (0 warnings, 0 errors).

[Copilot]

Pull request overview

Normalizes empty-array initialization across DotnetTest IPC serializers without changing behavior.

Changes:

• Initializes message arrays with [].
• Removes redundant ?? [] return fallbacks.

Show a summary per file

File	Description
CommandLineOptionMessagesSerializer.cs	Normalizes command-line message initialization.
FileArtifactMessagesSerializer.cs	Normalizes artifact message initialization.
TestResultMessagesSerializer.cs	Normalizes successful and failed result initialization.

Review details

• Files reviewed: 3/3 changed files
• Comments generated: 0
• Review effort level: Medium

[github-actions]

Note

🤖 Automated review by GitHub Copilot. Generated by the Expert Code Review workflow. To request a follow-up action, reply by tagging @copilot directly.

✅ 22/22 dimensions clean — no findings.

Summary: Clean mechanical normalization. The three files now consistently use = [] initialization (matching DiscoveredTestMessagesSerializer and TestInProgressMessagesSerializer), removing the mixed = null + ?? [] pattern. Verified:

• IPC Wire Compatibility — serialization is untouched; deserialization produces identical output (empty array fallback preserved via = [] initialization).
• Algorithmic Correctness — Read*Payload methods always return non-null arrays, so the variable is guaranteed non-null at the return site regardless of which ReadFields branches fire.
• Performance — [] for arrays compiles to Array.Empty<T>() (cached singleton), so no allocation concern even in the (unlikely) case the field isn't present in the stream.
• Constructor compatibility — FileArtifactMessages, TestResultMessages, and CommandLineOptionMessages all accept the passed values correctly (verified against record signatures in IPC/Models/).