chore: consolidate 9 Roslyn-versioned project duplicates

[thomhurst]

Summary

• Merges three near-identical Directory.Build.props PropertyGroup blocks into one that dynamically derives AssemblyName, RootNamespace, and RoslynVersion from the project name using MSBuild regex functions
• Consolidates Analyzer.props, AspNetCore.Analyzer.props, and SourceGenerator.props into a single unified Roslyn.props that uses the derived _BaseProjectName property to link source files from the correct base project
• Removes explicit RoslynVersion from all 9 .csproj files (now derived from project name suffix, e.g. Roslyn44 -> 4.4, Roslyn414 -> 4.14)
• Removes duplicated Folder items from SourceGenerator .csproj files (moved to shared props)

Result: Each Roslyn-versioned .csproj is now a minimal 3-line stub that only imports Roslyn.props. Adding a new Roslyn version target requires only creating a new directory with a stub .csproj -- no other files need updating.

Net change: 14 files changed, 39 insertions, 111 deletions (-72 lines). Three .props files deleted.

Closes #4883

Test plan

• All 9 Roslyn-versioned projects build successfully
• Downstream projects (TUnit.Core, TUnit.AspNetCore, TUnit.SourceGenerator.IncrementalTests) build successfully
• Full solution build passes with 0 errors
• Verified RoslynVersion, AssemblyName, and RootNamespace resolve correctly for all projects via dotnet msbuild -getProperty

[claude]

Code review

No issues found. Checked for bugs and CLAUDE.md compliance.

This is a well-executed MSBuild refactor. A few things worth calling out positively:

RoslynVersion derivation is correct. The two-step regex (Roslyn(\d+)$ → ^(\d) → $1.) correctly produces 4.4, 4.7, and 4.14 from the respective project name suffixes. The .NET Regex.Replace with ^ anchoring means "414" → "4.14" without risk of double-inserting a dot.

Condition specificity is solid. Using .Contains('.Roslyn') (with the dot) rather than .Contains('Roslyn') prevents any hypothetical false matches on projects that might have "Roslyn" as part of another word. The existing 9 variant projects are the only ones in the solution with .Roslyn in their names.

Behavior parity is maintained. The conditional ItemGroup structure in Roslyn.props correctly replicates the previous per-file behavior:

• EndsWith('.Analyzers') → EmbeddedResource for both TUnit.Analyzers and TUnit.AspNetCore.Analyzers ✓
• Contains('AspNetCore.Analyzers') → AdditionalFiles for release tracking only for AspNetCore ✓
• Contains('SourceGenerator') → Folder IDE link only for SourceGenerator ✓

The result is each Roslyn-versioned .csproj becoming a 3-line stub, and adding a new Roslyn version target will now require only creating a new directory with that stub — a clear DRY improvement with no behavioral regressions.