fix(train): load Nova hyperparameters file in ModelTrainer

[saisharan0103]

Fixes #5770.

ModelTrainer.from_recipe has a Nova-specific path that handles string hyperparameters values as file paths. That branch called cls._validate_and_load_hyperparameters_file(...), but ModelTrainer only defines _validate_and_fetch_hyperparameters_file(...). As a result, Nova recipes using a hyperparameters file path raised AttributeError before the file could be validated, parsed, and merged with recipe-derived hyperparameters.

This replaces the erroneous call with the existing _validate_and_fetch_hyperparameters_file helper.

A focused unit regression test now creates a temporary Nova recipe and temporary hyperparameters file, calls ModelTrainer.from_recipe with hyperparameters set to that file path, and asserts the parsed values are present in trainer.hyperparameters without raising AttributeError.

Existing dict-hyperparameters and non-Nova behavior are unchanged.

ruff check sagemaker-train/src/sagemaker/train/model_trainer.py sagemaker-train/tests/unit/train/test_model_trainer.py reports no new findings on the changed files.
Ran pytest -x locally with no new failures.