Refactor `TestNodeProperties.cs` (1295 lines) into focused files

[Evangelink]

Overview

The file src/Platform/Microsoft.Testing.Platform/Messages/TestNodeProperties.cs has grown to 1295 lines, making it harder to navigate and maintain. This task involves refactoring it into smaller, more focused files.

Current State

• File: src/Platform/Microsoft.Testing.Platform/Messages/TestNodeProperties.cs
• Size: 1295 lines
• Language: C#

Structural Analysis

The file defines many independent types all sharing the same namespace Microsoft.Testing.Platform.Extensions.Messages. They fall into distinct logical groups:

Group	Types
Base interface	IProperty
State properties	TestNodeStateProperty (abstract base), DiscoveredTestNodeStateProperty, InProgressTestNodeStateProperty, PassedTestNodeStateProperty, SkippedTestNodeStateProperty, FailedTestNodeStateProperty, ErrorTestNodeStateProperty, TimeoutTestNodeStateProperty, CancelledTestNodeStateProperty
Timing properties	TimingInfo, StepTimingInfo, TimingProperty
Location properties	LinePosition, LinePositionSpan, FileLocationProperty (abstract), TestFileLocationProperty
Identity/metadata	TestMethodIdentifierProperty, TestMetadataProperty
Output/artifact	StandardOutputProperty, StandardErrorProperty, FileArtifactProperty
Internal	SerializableKeyValuePairStringProperty

Refactoring Strategy

Proposed File Splits

Based on the file's structure, split it into the following focused files inside src/Platform/Microsoft.Testing.Platform/Messages/:

1. IProperty.cs

   • Contents: IProperty interface
   • Responsibility: Base contract for all test node properties

2. TestNodeStateProperties.cs

   • Contents: TestNodeStateProperty, DiscoveredTestNodeStateProperty, InProgressTestNodeStateProperty, PassedTestNodeStateProperty, SkippedTestNodeStateProperty, FailedTestNodeStateProperty, ErrorTestNodeStateProperty, TimeoutTestNodeStateProperty, CancelledTestNodeStateProperty
   • Responsibility: All test execution state properties (pass/fail/skip/timeout/etc.)

3. TimingProperties.cs

   • Contents: TimingInfo, StepTimingInfo, TimingProperty
   • Responsibility: Elapsed-time and per-step timing data attached to test nodes

4. FileLocationProperties.cs

   • Contents: LinePosition, LinePositionSpan, FileLocationProperty, TestFileLocationProperty
   • Responsibility: Source file location metadata for test nodes

5. TestMethodIdentifierProperty.cs

   • Contents: TestMethodIdentifierProperty
   • Responsibility: Strongly-typed identity of the test method (assembly, namespace, type, method, parameters)

6. TestMetadataProperty.cs

   • Contents: TestMetadataProperty
   • Responsibility: Arbitrary key/value metadata attached to a test node

7. OutputProperties.cs

   • Contents: StandardOutputProperty, StandardErrorProperty
   • Responsibility: Captured stdout/stderr output produced during a test run

8. FileArtifactProperty.cs

   • Contents: FileArtifactProperty
   • Responsibility: File artifacts (attachments) produced by a test

9. SerializableKeyValuePairStringProperty.cs (internal)

   • Contents: SerializableKeyValuePairStringProperty
   • Responsibility: Internal serialization helper property

Implementation Guidelines

1. Preserve Behavior: All existing functionality must work identically after the split
2. Maintain Public API: Keep all public symbols in the same namespace (Microsoft.Testing.Platform.Extensions.Messages) — no namespace changes needed
3. PublicAPI.Unshipped.txt: No changes needed as this is pure restructuring of existing public API
4. Test After Each Split: Run the test suite (./build.sh) after each incremental file move
5. One File at a Time: Split one logical group at a time to make review easier
6. Check PublicAPI.Shipped.txt: Confirm the API surface is unchanged — no entries should be added or removed

Acceptance Criteria

• TestNodeProperties.cs is replaced by the focused files listed above
• Each new file is under 300 lines
• All tests pass after refactoring
• No breaking changes to public API
• PublicAPI.Shipped.txt / PublicAPI.Unshipped.txt are unchanged

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Priority: Medium
Effort: Small (purely mechanical file splits — no logic changes required)
Expected Impact: Improved code navigability, easier per-type Git history, reduced merge conflicts on this file

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