fix(train): correct Networking field names in ModelTrainer intelligent defaults

[mandipat]

Summary

Fixes two bugs in _populate_intelligent_defaults_from_training_job_space() in model_trainer.py that prevent ModelTrainer from working when sagemaker_config has a VpcConfig set. This makes SageMaker distribution 4.0 unusable out of the box.

Bug 1 — Wrong field name passed to Networking constructor

default_enable_network_isolation does not exist as a field in sagemaker-core 2.x. The correct field name is enable_network_isolation. This caused a Pydantic ValidationError on every model_trainer.train() call when VPC config was present in sagemaker_config.

# BEFORE (broken)
self.networking = Networking(
    default_enable_network_isolation=default_enable_network_isolation,  # field does not exist
    ...
)

# AFTER (fixed)
self.networking = Networking(
    enable_network_isolation=default_enable_network_isolation,
    ...
)

Bug 2 — security_group_ids silently assigned wrong value

When updating an existing Networking object, security_group_ids was mistakenly assigned the value from TRAINING_JOB_SUBNETS_PATH instead of TRAINING_JOB_SECURITY_GROUP_IDS_PATH.

# BEFORE (broken)
if self.networking.security_group_ids is None:
    self.networking.subnets = self.config_mgr.resolve_value_from_config(  # wrong attribute + wrong path
        config_path=TRAINING_JOB_SUBNETS_PATH
    )

# AFTER (fixed)
if self.networking.security_group_ids is None:
    self.networking.security_group_ids = self.config_mgr.resolve_value_from_config(
        config_path=TRAINING_JOB_SECURITY_GROUP_IDS_PATH
    )

Testing

Reproduced and verified the fix using a clean venv with sagemaker-core==2.10.0 and sagemaker-train==1.10.0 (exact versions shipped in SageMaker distribution 4.0).

from sagemaker.core.training.configs import Networking

# Bug 1 confirmed
try:
    Networking(default_enable_network_isolation=None)  # ValidationError
except Exception as e:
    print(f"BUG: {e}")

# Fix confirmed
Networking(enable_network_isolation=False)  # Works

Closes #5766