[tmva][sofie] throw on invalid BatchNormalization input count

[harz05]

Changes:

ParseBatchNormalization only constructed the operator when input_size() == 5 but had no else clause, so nodes with fewer inputs silently returned a null unique_ptr and crashed later in Generate() with an unrelated error message.

I've simply added an else throw to reject invalid input counts immediately at parse time with a clear error message, consistent with other parsers in the codebase.

Checklist:

• tested changes locally
• updated the docs (if necessary)

This PR fixes #21759

[harz05]

I'd also like to add a regression test for this(verifying Parse() throws std::runtime_error) but im not sure what's the best to add it.
TestCustomModelsFromONNX.cxx doesn't currently link against ROOTTMVASofieParser and placing an invalid .onnx in input_models/ would break the emitFromONNX fixture.
@guitargeek