fix: Table preview - dimension fallback and API validation

[sentry]

This PR addresses the issue DALGO-BACKEND-1VW where table chart previews failed with "No dimensions found after normalization!" when dimensions, dimension_col, and extra_dimension were all None in the payload.

Root Cause:

1. The normalize_dimensions function for table charts only checked payload.dimensions and did not implement the intended fallback to dimension_col or extra_dimension when payload.dimensions was empty or None.
2. The /api/charts/chart-data-preview/ endpoint lacked early input validation to reject payloads for table charts that had no dimension fields specified, leading to a ValueError further down the call stack.

Changes Made:

1. ddpui/core/charts/charts_service.py (normalize_dimensions): Implemented the fallback logic for table charts. If payload.dimensions is empty or None, it now checks payload.dimension_col and payload.extra_dimension to populate the dimensions list, aligning with the function's docstring and behavior for other chart types.
2. ddpui/api/charts_api.py (get_chart_data_preview): Added early validation. For table chart previews, if no dimension fields (dimensions, dimension_col, extra_dimension) are provided in the payload, an HttpError(400) is now raised with a clear message, preventing the request from proceeding with invalid input.
3. ddpui/core/charts/charts_service.py (get_chart_data_table_preview): Downgraded the logger.error to logger.warning for the ValueError raised when no dimensions are found. This is because the API layer now handles this validation upfront with a 400 response, making the ValueError an expected outcome for invalid input rather than an unhandled error.

Fixes DALGO-BACKEND-1VW