fix: normalize_dimensions fallback for table charts

[sentry]

This PR addresses issue DALGO-BACKEND-25A, where table charts would fail with a ValueError: At least one dimension is required for table charts if payload.dimensions was an empty list, even when payload.dimension_col was provided.

The root cause was identified in ddpui/core/charts/charts_service.py within the normalize_dimensions() function. For chart_type='table', the logic correctly prioritized payload.dimensions but failed to implement the documented fallback to payload.dimension_col and payload.extra_dimension when payload.dimensions was empty.

This fix adds the necessary fallback logic within the table chart type branch of normalize_dimensions(). Now, if payload.dimensions is empty, the function will correctly append payload.dimension_col and payload.extra_dimension to the final dimensions list, resolving the ValueError.

Fixes DALGO-BACKEND-25A