[file-diet] Refactor CiRunSummaryAggregation.cs (834 lines) into focused modules

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Overview

The file src/Platform/SharedExtensionHelpers/CiRunSummaryAggregation.cs has grown to 834 lines, mixing several distinct concerns: data models, coverage summary computation/rendering, aggregation logic, and fragment file I/O. This makes it harder to navigate and maintain.

(Note: the repository's single largest source file, src/Platform/Microsoft.Testing.Extensions.TrxReport/Hashing/XxHashShared.cs at 925 lines, is explicitly a verbatim port of dotnet/runtime's XxHashShared.cs and should not be restructured, since that would break the ability to diff/sync against upstream. This issue targets the next-largest file that is a genuine internal implementation.)

Current State

• File: src/Platform/SharedExtensionHelpers/CiRunSummaryAggregation.cs
• Size: 834 lines
• Language: C#

Structural Analysis

The file currently contains:

• Plain DTO/model classes: CiRunSummaryModule, CiRunSummaryTest, CiRunSummaryFailingClass, CiCoverageSummaryData, CiCoverageMetric, CiCoverageThreshold, CiRunSummaryAggregate, CiRunSummaryFragment.
• CiCoverageSummary (static class, ~130 lines) — coverage aggregation and Markdown rendering (AppendMarkdown).
• CiRunSummaryAggregation (static partial class, ~390 lines) — the core logic: CreateModule, WriteFragmentAsync, ValidateModule, private helpers WriteTextAsync/WriteAtomicAsync for atomic fragment file writes, plus the nested CiRunSummaryJsonContext JSON source-generation context.

Refactoring Strategy

Proposed File Splits

1. CiRunSummaryModels.cs

   • Contents: CiRunSummaryModule, CiRunSummaryTest, CiRunSummaryFailingClass, CiCoverageSummaryData, CiCoverageMetric, CiCoverageThreshold, CiRunSummaryAggregate, CiRunSummaryFragment
   • Responsibility: Pure data-transfer objects with no behavior, used across the aggregation pipeline.

2. CiCoverageSummary.cs

   • Contents: CiCoverageSummary static class (Aggregate, AppendMarkdown, and related private helpers)
   • Responsibility: Coverage-specific aggregation and Markdown report rendering, isolated from run/test summary logic.

3. CiRunSummaryAggregation.cs (trimmed)

   • Contents: CiRunSummaryAggregation partial class — CreateModule, WriteFragmentAsync, ValidateModule
   • Responsibility: Core orchestration of building and validating a module summary from test records.

4. CiRunSummaryFragmentIo.cs

   • Contents: WriteTextAsync, WriteAtomicAsync (as another partial of CiRunSummaryAggregation, or a small internal helper class), plus CiRunSummaryJsonContext
   • Responsibility: Atomic file I/O for writing fragment JSON to disk, decoupled from aggregation logic.

Implementation Guidelines

1. Preserve Behavior: All existing functionality must work identically after the split.
2. Maintain Public API: Keep exported/public and internal symbols accessible with the same names and namespace (Microsoft.Testing.Extensions).
3. Update Imports: Fix all import paths/usages throughout the codebase that reference these types.
4. Test After Each Split: Run the relevant unit/integration test suite after each incremental change.
5. One File at a Time: Split one module at a time to make review easier — start with the DTOs (lowest risk), then coverage summary, then fragment I/O.

Acceptance Criteria

• Original file is split into focused modules
• Each new file is under 300 lines
• All tests pass after refactoring
• No breaking changes to public API
• All import paths updated correctly

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Priority: Medium
Effort: Medium
Expected Impact: Improved code navigability, easier testing, reduced merge conflicts

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• expires on Aug 26, 2026, 7:07 PM UTC