refactor: refactor script execution and step validation

- Refactor `_ValidateScriptExecOptions` to its own method
- Add `_checkStepSeparation` method for step validation

packages/melos/lib/src/commands/run.dart

@@ -21,14 +21,12 @@ mixin _RunMixin on _Melos {
     }
 
     if (script.steps != null && script.steps!.isNotEmpty) {
-      if (script.exec != null) {
-        throw ScriptExecOptionsException._(
-          scriptName,
-        );
-      }
+      _validateScriptExecOptions(script, scriptName);
 
       _detectRecursiveScriptCalls(script);
 
+      _checkStepSeparation(script);
+
       await _runMultipleScripts(
         script,
         global: global,
@@ -70,6 +68,14 @@ mixin _RunMixin on _Melos {
     resultLogger.child(successLabel);
   }
 
+  void _validateScriptExecOptions(Script script, String scriptName) {
+    if (script.exec != null) {
+      throw ScriptExecOptionsException._(
+        scriptName,
+      );
+    }
+  }
+
   /// Detects recursive script calls within the provided [script].
   ///
   /// This method recursively traverses the steps of the script to check
@@ -326,6 +332,14 @@ mixin _RunMixin on _Melos {
     }
     logger.newLine();
   }
+
+  void _checkStepSeparation(Script script) {
+    final invalidSteps =
+        script.steps!.where((step) => step.contains('&&')).toList();
+    if (invalidSteps.isNotEmpty) {
+      throw InvalidStepFormatException._(invalidSteps.join(', '));
+    }
+  }
 }
 
 class NoPackageFoundScriptException implements MelosException {
@@ -421,3 +435,16 @@ class RecursiveScriptCallException implements MelosException {
         'loop.';
   }
 }
+
+class InvalidStepFormatException implements MelosException {
+  InvalidStepFormatException._(this.stepName);
+
+  final String stepName;
+
+  @override
+  String toString() {
+    return 'InvalidStepFormatException: The step $stepName'
+        'contains "&&". Split it into separate steps to enhance clarity and '
+        'error handling.';
+  }
+}