Wire val_size through CLI and pipeline construction

[Copilot]

Description

Daily audit found a split-configuration gap: HyperionConfig.val_size existed but was not reachable from CLI/Makefile and was not propagated into pipeline construction. This made validation split control effectively non-functional at runtime.

Changes in this pull request

• CLI + config propagation

  • Added --val-size to src/main.py argument parsing.
  • Mapped parsed value into HyperionConfig(val_size=...).
  • Passed config.val_size into both StackedModelTrainingPipeline and TimeSeriesStackedModelTrainingPipeline.

• Makefile runtime parity

  • Added VAL_SIZE ?= 0.1.
  • Included --val-size $(VAL_SIZE) in _RUN_ARGS so make run can control validation split.

• Regression coverage

  • Added tests/test_main.py to assert:
    • default val_size is preserved when no CLI override is provided,
    • custom --val-size is parsed and applied.

parser.add_argument(
    "--val-size",
    type=float,
    default=defaults.val_size,
    help="Fraction of data reserved for validation (0 < val_size < 1); train_size = 1 - test_size - val_size.",
)