refactor: Consolidate NotInParallel and shell test patterns

[thomhurst]

Summary

• Creates NotInParallelTestModule base class in TestHelpers for NotInParallel tests
• Creates NotInParallelTracker using ConcurrentDictionary (avoids the ConcurrentBag.TryTake flaky issue)
• Creates ModuleResultAssertions.AssertCommandOutput() for shell test assertions
• Updates BashTests, CmdTests, PowershellTests to use shared assertion helper
• Updates all NotInParallel test files to use shared base class and tracker

Fixes #1592, #1564

Test plan

• Build passes
• Tests pass

🤖 Generated with Claude Code

[thomhurst]

Summary

Refactors test code to reduce duplication by introducing assertion helpers and base classes for NotInParallel tests, while fixing a concurrency bug in the tracking mechanism.

Critical Issues

None found ✅

This PR actually fixes a bug in the original code where ConcurrentBag.TryTake() could remove an arbitrary item instead of the specific module being tracked. The fix correctly uses ConcurrentDictionary<string, bool> instead.

Suggestions

NotInParallelTestModule.cs:90 - Minor optimization opportunity

The Reset() method uses a while loop to clear violations:

while (_violations.TryTake(out _)) { }

Since ConcurrentBag<T> doesn't have a Clear() method until .NET 5+, this is the correct approach for compatibility. If the project targets .NET 5+, consider documenting this or using the built-in Clear() method for better readability.

General observation

The refactoring significantly improves code maintainability by:

• Eliminating ~200 lines of duplicated assertion code
• Providing reusable base classes for parallel execution testing
• Making test intent clearer through helper methods

Verdict

✅ APPROVE - No critical issues. The PR improves code quality and fixes a concurrency bug.