Refactor plugin lifecycle cleanup in PluginContext

[Ayrton09]

Summary

This PR improves plugin lifecycle cleanup in PluginContext by making disposal explicit, idempotent, and centralized.

Why

PluginContext currently owns several long-lived resources during a plugin's lifetime:

• the plugin instance
• the plugin DI scope
• the plugin root ServiceProvider
• the plugin loader
• hot-reload related event subscriptions
• a FileSystemWatcher when hot reload is enabled

At the moment, unload/cleanup behavior is not fully centralized, which makes repeated unload/reload cycles harder to reason about and increases the risk of leaked resources.

This PR keeps the scope intentionally small and only focuses on PluginContext resource ownership and cleanup.

Changes

• Implement IDisposable on PluginContext
• Make owned-resource cleanup explicit and idempotent
• Dispose the plugin file watcher cleanly
• Unsubscribe Loader.Reloaded before disposing the loader
• Dispose the root ServiceProvider
• Replace the NotImplementedException used in self-termination with PluginTerminationException

Non-goals

This PR does not:

• change PluginManager ownership rules
• refactor css_plugins unload/reload
• change bootstrap/host builder behavior
• redesign contextual dependency resolution

Those changes are better handled in follow-up PRs.

Testing

• Load a plugin normally
• Unload a plugin once and verify no exceptions
• Reload the same plugin multiple times
• Replace/delete the plugin DLL and confirm hot-reload/unload still behaves correctly
• Verify no watcher / file lock regressions

Files to review

• managed/CounterStrikeSharp.API/Core/Plugin/PluginContext.cs