feat(core): improve error handling for not found plugin assemblies

[DevD4v3]

Summary

Improve the error handling when a configured plugin assembly cannot be found.

Instead of propagating the generic exception produced by AssemblyDependencyResolver, CPlugin.Net now validates that the plugin assembly exists before attempting to load it.

Changes

• Add PluginNotFoundException.
• Validate the plugin assembly path before creating AssemblyDependencyResolver.
• Include the expected plugin path in the exception message.
• Add tests covering the new behavior.

Example

Before:

System.InvalidOperationException: Failed to locate managed application [C:\...\Example.EconomyPlugin.dll].
   at System.Runtime.Loader.AssemblyDependencyResolver..ctor(String componentAssemblyPath)
   at CPlugin.Net.PluginLoadContext..ctor(String pluginPath)
   at CPlugin.Net.PluginLoader.Load(...)

After:

The plugin 'Example.AppointmentPlugin.dll' was not found at:

C:\...\plugins\Example.AppointmentPlugin\Example.AppointmentPlugin.dll

Ensure the plugin project has been built and the assembly exists in the plugins directory.