feat: add pkl file validation

src/crewai/cli/train_crew.py

@@ -16,6 +16,9 @@ def train_crew(n_iterations: int, filename: str) -> None:
         if n_iterations <= 0:
             raise ValueError("The number of iterations must be a positive integer.")
 
+        if filename.endswith(".pkl"):
+            raise ValueError("The filename must not end with .pkl")
+
         result = subprocess.run(command, capture_output=False, text=True, check=True)
 
         if result.stderr: