feat(tools): add code coverage tools #104

Part of #95. Depends on #101 for run-completion tracking.

Add code coverage tools — trigger an analysis run and read the results back.

Proposed tools

Triggering a run

DTE.ExecuteCommand("Test.AnalyzeCodeCoverageForAllTests"), and Test.CodeCoverageResults to surface the window.

Reuse the IOperationState completion tracking from #101 — VS 2026 raises an AnalyzeCodeCoverageOperationStateChanged signal, and the run needs to finish before results are readable.

Reading results — two routes

Route A — VS 2026 brokered service (native, but limited)

Microsoft.CodeCoverage.VisualStudio.Contracts.dll:

Task<CoverageRunData> IReportService.GetReportAsync(string inputPath, CancellationToken ct);
Task<string[]>        IReportService.MergeCoverageFilesAsync(string outputPath, string[] files, MergeOperation op, CancellationToken ct);

class CoverageFileData { string Path; int LinesCovered; int LinesNotCovered; int LinesPartiallyCovered; LineCoverageData[] LinesCoverageData; }
class LineCoverageData { LineCoverageStatus CoverageStatus; int StartLine; int StartColumn; int EndLine; int EndColumn; int BlocksCovered; }
enum  LineCoverageStatus { Covered, NotCovered, PartiallyCovered }

Drawbacks: VS 2026 only (the assembly doesn't exist in VS 2022), granularity is per-file and per-line only — no class or module rollup — and unlike the TestWindow and Terminal assemblies it has no binding redirect or codeBase in devenv.exe.config, so resolution needs handling ourselves.

Route B — dotnet-coverage / Microsoft.CodeCoverage.Console (portable, recommended)

Convert .coverage → Cobertura and parse. Works on both VS versions, no dependency on unsupported internal assemblies, and gives a proper module → class → method hierarchy, which is what was actually asked for in #95.

Leaning toward B as the primary implementation, with A as a possible optimization on VS 2026. Open to input.

Edition gating

Code coverage was Enterprise-only through VS 2022; it became available in Community and Professional in VS 2026. Verified on disk — CommonExtensions\Microsoft\AnalyzeCodeCoverage\ exists in VS 2026 Community but not VS 2022 Community.

The tools must degrade gracefully: detect availability and return a clear "code coverage is not available in this edition of Visual Studio" message rather than failing opaquely or hanging on a disabled command. DTE.ExecuteCommand throws when the command is disabled, so that needs catching explicitly.

Out of scope

A class/module coverage tree read directly from the VS API — that granularity doesn't exist in the contracts. Route B covers it instead.