Fix startup crash when published as self-contained single-file executable

[m-sekhon]

Problem

PluginLoader.Load() derived its plugin search directory from Assembly.GetEntryAssembly().Location:

_executingDir = Directory.GetParent(Assembly.GetEntryAssembly().Location).FullName;

.NET leaves Location empty ("") when the app is published as a self-contained, single-file executable (-p:PublishSingleFile=true --self-contained true). Directory.GetParent("") then throws, crashing the app before it can even start:

Unhandled exception. System.ArgumentException: The value cannot be an empty string. (Parameter 'path')
   at System.ArgumentException.ThrowNullOrEmptyException(String argument, String paramName)
   at System.IO.Directory.GetParent(String path)
   at BotSharp.Core.Plugins.PluginLoader.Load(Action`1 loaded, String plugin)
   at BotSharp.Core.BotSharpCoreExtensions.RegisterPlugins(IServiceCollection services, IConfiguration config)
   at BotSharp.Core.BotSharpCoreExtensions.AddBotSharpCore(IServiceCollection services, IConfiguration config, Action`1 configOptions)
   at Program.<Main>$(String[] args)

Fixes #669

Fix

Use AppContext.BaseDirectory instead, which resolves correctly for both regular and single-file-published deployments.

Verification

• Reproduced the original crash by publishing with dotnet publish -r linux-x64 --self-contained true -p:PublishSingleFile=true and running the resulting binary directly — got the exact stack trace from Publish exception #669.
• Confirmed the fix resolves it — same publish/run steps after the change start the app cleanly instead of crashing.
• dotnet run (the documented dev workflow) verified unaffected before and after.
• Added two regression tests in tests/BotSharp.Core.UnitTests/Plugins/PluginLoaderTests.cs:
  • asserts Load() doesn't throw and that the resolved executing directory matches AppContext.BaseDirectory
  • asserts it can still find and load a real plugin assembly from that directory
• Full BotSharp.Core.UnitTests suite passes: 376/376.

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