test(tuner): Add unit tests for driver/code channel building

Add 25 unit tests covering the tuner changes from PR #5634:
- _prepare_model_trainer_for_tuning guard logic
- _build_driver_and_code_channels sm_drivers channel creation
- _build_training_job_definition _tuner_channels inclusion
- Environment and VPC config passthrough
- sourcedir.tar.gz upload and sagemaker_submit_directory HP
- static_hyperparameters getattr fallback

Author: nargokul