chore: centralize environment variable constants (#4882)

[thomhurst]

Summary

• Introduces TUnit.Engine/Configuration/EnvironmentConstants.cs containing all environment variable names used across the engine as const string fields
• Replaces 15 scattered magic string literals in 7 files with references to the new constants class
• Groups constants by purpose: reporters, execution, display/diagnostics, JUnit output, legacy/deprecated, and external CI variables

Closes #4882

Test plan

• dotnet build TUnit.Engine/TUnit.Engine.csproj passes across all target frameworks (net8.0, net9.0, net10.0, netstandard2.0) with zero warnings and zero errors
• Verify no behavioral changes — all constants map to the exact same string values as before
• Confirm CI passes end-to-end

[claude]

Code review

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

This is a clean, well-executed refactoring. All 15 environment variable constants map exactly to their original string literals, and the new EnvironmentConstants class is appropriately scoped as internal static. The PR improves maintainability by eliminating magic strings scattered across 7 files without introducing any behavioral changes.