fix: reject non-dict values in process_config with clear ValidationError

Fixes #4419. When an invalid type (e.g. a string) was passed for a field
that expects a Pydantic model like BaseAgent, the process_config function
would crash with AttributeError: 'str' object has no attribute 'get'.

Now process_config raises ValueError for non-dict inputs, which Pydantic
wraps into a proper ValidationError with a clear message like:
  'BaseAgent expected a mapping/dictionary, got str'

Co-Authored-By: João <joao@crewai.com>

lib/crewai/src/crewai/utilities/config.py

@@ -14,7 +14,16 @@ def process_config(
 
     Returns:
         The updated values dictionary.
+
+    Raises:
+        ValueError: If values is not a dictionary (e.g. when an invalid type
+            is passed for a field that expects a Pydantic model).
     """
+    if not isinstance(values, dict):
+        raise ValueError(
+            f"{model_class.__name__} expected a mapping/dictionary, "
+            f"got {type(values).__name__}"
+        )
     config = values.get("config", {})
     if not config:
         return values

lib/crewai/tests/test_task.py

@@ -1748,3 +1748,50 @@ def test_async_execution_fails():
       with pytest.raises(RuntimeError, match="boom!"):
         execution = task.execute_async(agent=researcher)
         execution.result()
+
+
+def test_task_with_invalid_agent_string_raises_validation_error():
+    with pytest.raises(ValidationError) as exc_info:
+        Task(
+            description="Test task",
+            expected_output="Test output",
+            agent="not_an_agent",
+        )
+    assert "BaseAgent expected a mapping/dictionary, got str" in str(exc_info.value)
+
+
+def test_task_with_invalid_agent_int_raises_validation_error():
+    with pytest.raises(ValidationError) as exc_info:
+        Task(
+            description="Test task",
+            expected_output="Test output",
+            agent=123,
+        )
+    assert "BaseAgent expected a mapping/dictionary, got int" in str(exc_info.value)
+
+
+def test_task_with_invalid_agent_list_raises_validation_error():
+    with pytest.raises(ValidationError) as exc_info:
+        Task(
+            description="Test task",
+            expected_output="Test output",
+            agent=[1, 2, 3],
+        )
+    assert "BaseAgent expected a mapping/dictionary, got list" in str(exc_info.value)
+
+
+def test_task_with_none_agent_is_valid():
+    task = Task(
+        description="Test task",
+        expected_output="Test output",
+        agent=None,
+    )
+    assert task.agent is None
+
+
+def test_task_without_agent_is_valid():
+    task = Task(
+        description="Test task",
+        expected_output="Test output",
+    )
+    assert task.agent is None