Fix missing --detailed-stacktrace option since v0.50.0

[Copilot]

The --detailed-stacktrace command line option was inadvertently broken in v0.50.0 when the verbosity system was unified. The DetailedStacktraceCommandProvider was removed from registration but the TUnitMessageBus still tried to check for the option, causing it to never work.

Root Cause

In TestApplicationBuilderExtensions.cs, the comment indicated that the unified VerbosityCommandProvider "replaces HideTestOutput, DisableLogo, DetailedStacktrace", and the DetailedStacktraceCommandProvider registration was removed. However, the TUnitMessageBus.SimplifyStacktrace() method continued to check for DetailedStacktraceCommandProvider.DetailedStackTrace, which would always return false since the provider was no longer registered.

Solution

This PR restores the --detailed-stacktrace functionality while maintaining the new unified verbosity system:

1. Re-registered DetailedStacktraceCommandProvider for backward compatibility
2. Enhanced TUnitMessageBus to check both the legacy --detailed-stacktrace flag and the new VerbosityService.ShowDetailedStackTrace property
3. Added null-safety to prevent potential NullReferenceException

Behavior After Fix

• --detailed-stacktrace: Works as before (backward compatible)
• --verbosity verbose or --verbosity debug: Also enables detailed stack traces (unified approach)
• --verbosity normal or --verbosity minimal: Standard behavior (no detailed traces)

Both approaches work independently or together, providing full backward compatibility while supporting the modern verbosity system.

Testing

• Verified PublicAPI tests pass (no breaking changes)
• Confirmed command line provider registration is correct
• Validated the logic handles both flag approaches properly

Fixes #2737.

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