Fix HyperparameterTuner to include ModelTrainer internal channels (aws#5516)

* Fix HyperparameterTuner to include ModelTrainer internal channels

HyperparameterTuner._build_training_job_definition() was only copying
user-provided input channels, missing ModelTrainer's internal channels
(code, sm_drivers). This caused tuning jobs to fail when using custom
training scripts.

- Add logic to include ModelTrainer's input_data_config channels
- Refactor test_tuner.py with factory functions to reduce duplication
- Add test for internal channels inclusion

Fixes aws#5508

* fix formatting

* Fix static_hyperparameters initialization and reference

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Co-authored-by: adishaa <adishaa@amazon.com>
Co-authored-by: Sharma <adishaa@14147d9496bd.ant.amazon.com>

Author: 3 people