fix: Return 400 for missing metrics in chart data preview

[sentry]

This PR addresses the issue where ValueError: At least one metric is required for aggregated charts was being reported as a 500 error to Sentry (DALGO-BACKEND-1W4).

Problem:
The /api/charts/chart-data-preview/ and /api/charts/chart-data-preview/total-rows/ endpoints were catching ValueError (raised when a non-table chart type was requested without metrics) with a broad except Exception block. This resulted in an HTTP 500 response and an unnecessary error log in Sentry, treating a client-side input validation issue as a server error.

Solution:

1. Modified get_chart_data_preview to specifically catch ValueError before the general Exception handler.
2. Modified get_chart_data_preview_total_rows to also specifically catch ValueError.
3. For ValueError instances, an HTTP 400 (Bad Request) is now returned to the client, providing a more accurate response for invalid input.
4. The logging level for these ValueError cases has been downgraded from error to warning to prevent them from being reported as critical issues in Sentry.

This change aligns the behavior of the chart data preview endpoints with the existing get_chart_data endpoint, which already handles ValueError in a similar manner.

Fixes DALGO-BACKEND-1W4

[sentry]

Codecov Report

❌ Patch coverage is 0% with 8 lines in your changes missing coverage. Please review.
✅ Project coverage is 58.64%. Comparing base (75cb6fd) to head (9a2ec81).

Files with missing lines	Patch %	Lines
ddpui/api/charts_api.py	0.00%	8 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1376      +/-   ##
==========================================
- Coverage   58.66%   58.64%   -0.03%     
==========================================
  Files         132      132              
  Lines       15690    15697       +7     
==========================================
  Hits         9205     9205              
- Misses       6485     6492       +7     

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