Function validateAndNormalizeConfig

Validate and normalise a ForecastConfig against a ModelConfig.

Returns a new object; the input is never mutated.

Adjustments applied (matching the Python compile() logic):

• maxContext rounded up to the next multiple of inputPatchLen.
• maxHorizon rounded up to the next multiple of outputPatchLen.
• If useContinuousQuantileHead is true, maxHorizon must be ≤ outputQuantileLen.