fix(api): Return 400 for preference validation ValueErrors

[sentry]

Previously, ValueError exceptions raised during preference validation (e.g., a positive weight for a shift count preference with |x - T|^2 expression) were caught by the generic except Exception block in core/nurse_scheduling/serve.py. This resulted in an HTTP 500 server error and an error-level log in Sentry, despite the issue being a client-side input validation failure.

This change introduces a specific except ValueError block in the /optimize-and-export-xlsx handler. It now catches these validation-related ValueErrors, logs them at a warning level, and returns an HTTP 400 Bad Request to the client. This aligns the handling of these errors with other client-side validation issues (like NotImplementedError and ValidationError), providing clearer feedback to the user and preventing unnecessary server error reports.

Fixes NURSE-SCHEDULING-16

[codecov]

Codecov Report

❌ Patch coverage is 33.33333% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 92.06%. Comparing base (69e9288) to head (2c4052f).

Files with missing lines	Patch %	Lines
core/nurse_scheduling/serve.py	33.33%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##              dev      #23      +/-   ##
==========================================
- Coverage   92.12%   92.06%   -0.06%     
==========================================
  Files          46       46              
  Lines        3300     3303       +3     
  Branches      458      458              
==========================================
+ Hits         3040     3041       +1     
- Misses        161      163       +2     
  Partials       99       99              

Flag	Coverage Δ
core	95.84% <33.33%> (-0.11%)	⬇️
web-frontend	88.01% <ø> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

Files with missing lines	Coverage Δ
core/nurse_scheduling/serve.py	83.54% <33.33%> (-1.99%)	⬇️

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