refactor: consolidate error handling with fail() function in session handlers

- Replace repetitive error handling code with fail() function in HandleGlobalRequest
- Standardize error response pattern across all handler methods
- Improve code maintainability and reduce duplication

internal/port/port.go

@@ -74,7 +74,6 @@ func (pm *manager) GetUnassignedPort() (uint16, bool) {
 
 	for _, port := range pm.sortedPorts {
 		if !pm.ports[port] {
-			pm.ports[port] = true
 			return port, true
 		}
 	}