Split GitHub Actions summary reporter into focused partials

[Amaury Levé (Evangelink)]

GitHubActionsSummaryReporter.cs had grown difficult to navigate because it combined event ingestion, failure retention, snapshot construction, and step-summary writing in one file. This refactor separates those responsibilities while preserving behavior and API surface.

Changes

• Move failure retention, ordering, and diagnostic capture into a focused partial.
• Move immutable snapshot and aggregation-module construction into a focused partial.
• Move test-session finishing and step-summary write orchestration into a focused partial.
• Keep the core reporter and every new split file below 300 lines.

Validation

• Built the GitHub Actions reporter for all target frameworks with zero warnings.
• Passed all 95 GitHubActionsSummaryReporterTests.
• Passed the full extensions unit-test suite: 1,490 passed and 7 platform-specific skips.

Fixes #10848

[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.

This is a pure mechanical refactor that splits GitHubActionsSummaryReporter.cs into three focused partial class files (FailureTracking, Snapshot, SummaryWriting). The code is moved verbatim — no logic, API surface, or behavioral changes. The using Microsoft.Testing.Platform; directive removed from the main file is correctly placed in the partials that reference its types. Clean split with no issues.

[Copilot]

Copilot review overview

Review tier: Balanced
Findings: None

What changed in this PR

Refactors the GitHub Actions summary reporter into focused partial classes without changing behavior or API surface.

Changes:

• Separates failure tracking, snapshot construction, and summary writing.
• Reduces every reporter file below 300 lines.
• Preserves existing lifecycle and aggregation logic.

File	Description
GitHubActionsSummaryReporter.cs	Retains core wiring and event ingestion.
GitHubActionsSummaryReporter.FailureTracking.cs	Contains failure retention and diagnostics.
GitHubActionsSummaryReporter.Snapshot.cs	Contains snapshot and module construction.
GitHubActionsSummaryReporter.SummaryWriting.cs	Contains session completion and summary writing.

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