fix: add training_plan_arn to compute MagicMock in serverful tests (#6270)

The training_plan_arn passthrough fix causes Pydantic validation errors
in tests that use MagicMock for the compute object without explicitly
setting training_plan_arn. MagicMock returns a MagicMock object instead
of None/string, which Pydantic rejects.

Add training_plan_arn=None to the compute MagicMock in
test_base_trainer_serverful.py and test_serverful_recipe_validation.py.

Author: mrpic-amzn

sagemaker-train/tests/unit/train/test_base_trainer_serverful.py

@@ -31,6 +31,7 @@ def __init__(self, **kwargs):
             instance_count=1,
             volume_size_in_gb=100,
             keep_alive_period_in_seconds=None,
+            training_plan_arn=None,
         )
         self.networking = None
         self.stopping_condition = None

sagemaker-train/tests/unit/train/test_serverful_recipe_validation.py

@@ -41,6 +41,7 @@ def __init__(self, **kwargs):
             instance_count=4,
             volume_size_in_gb=100,
             keep_alive_period_in_seconds=None,
+            training_plan_arn=None,
         )
         self.networking = None
         self.stopping_condition = None