chore: raise phpstan to level 8 and fix null safety

- Updated phpstan.neon.dist to level 8
- Added null checks for ReflectionClass::getConstructor() calls in test files
- Enhanced PluginSession to handle nullable instanceId parameter
- Used PHP 8+ null coalescing throw operator for clean error handling
- Improved null safety across test and source files

Key improvements:
- All ReflectionMethod calls now safely handle null returns
- deleteInstance method properly validates required instanceId parameter
- Modern PHP syntax for null safety with descriptive error messages
- No breaking changes to existing functionality

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <[email protected]>

Author: Ninerian

src/RemoteCall/DeleteInstanceTrait.php

@@ -17,7 +17,7 @@ protected function exitRemoteCall(): void
         exit;
     }
 
-    private function deleteInstance(string $instanceId, RemoteCallInterface $remoteCallHandler): void
+    private function deleteInstance(string $instanceId, RemoteCallInterface $remoteCallHandler): never
     {
         if ($remoteCallHandler instanceof DeleteInstanceCallHandlerInterface) {
             $result = $remoteCallHandler->deleteInstance($instanceId);