Fix benchmarks actually measuring NoopLoggerProvider instead of configured loggers

[Copilot]

BuildLogger unconditionally called ClearProviders() then injected NoopLoggerProvider, wiping out every registered logging provider. Every benchmark — NLog, Log4Net, Serilog — was measuring identical no-op overhead, not the actual logger pipelines.

Changes

• BuildLogger: Remove the ClearProviders() + NoopLoggerProvider override; let each benchmark use its own registered provider.
• MS Logger baseline: Apply NoopLoggerProvider explicitly in the MS logger setup only — it's the intended noop baseline.
• NLog: Replace LoadConfigurationFromFile("nlog.config") with an empty LoggingConfiguration — NLog pipeline active, no file I/O:

  s.AddLogging(b => b.AddNLog(new NLog.Config.LoggingConfiguration()));

• Log4Net: Replace XmlConfigurator file-based config with a programmatic MemoryAppender — Log4Net pipeline active, no file I/O.
• Cleanup: Remove now-unused using log4net.Config and NLogManager alias.