Add dataarray size details to merge errors

[pragnyanramtha]

Problem summary
create_dataset reports a generic merge failure when target data arrays cannot align, but it does not show the per-target sizes that explain mismatched coordinates.

Root cause
The error path only included merged coordinate dumps, which make it hard to compare original input data arrays by target.

Files changed

• mllam_data_prep/create_dataset.py
• tests/test_dataset.py

Tests run

• python -m pytest tests/test_dataset.py::test_merge_error_includes_dataarray_sizes_by_target -q
• python -m pytest tests/test_dataset.py -q

Risk notes
Low risk. The change only adds detail to an existing invalid-config exception path and keeps successful merge behavior unchanged.

Closes #105

[Copilot]

Pull request overview

Adds per-target DataArray size details to the InvalidConfigException raised when _merge_dataarrays_by_target fails, making mismatched dimensions easier to diagnose. Also adds a unit test that triggers the error path and verifies the formatted output.

Changes:

• New helper _format_dataarray_sizes_by_target builds a multi-line summary of dims/sizes per target.
• The merge failure exception message now includes the formatted size summary.
• Added test_merge_error_includes_dataarray_sizes_by_target covering the new error content.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
mllam_data_prep/create_dataset.py	Adds size-summary helper and includes it in the merge-failure exception message.
tests/test_dataset.py	Adds a test asserting the new size details appear in the exception message.

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