fix: Fix BeforeValidator treating 0 as falsy in configuration fields

[vdusek]

Summary

• The BeforeValidator(lambda val: val or None) on max_paid_dataset_items and max_total_charge_usd configuration fields treated 0 as falsy, converting it to None. This meant a legitimate value of 0 (e.g. a zero charge limit) would be silently discarded.
• Replaced with lambda val: val if val != '' else None which only converts empty strings (from unset env vars) to None, preserving 0 as a valid value.

Test plan

• Added unit test verifying max_paid_dataset_items=0 is preserved (not converted to None)
• Added unit test verifying max_total_charge_usd=0 is preserved (not converted to None)
• Added unit test verifying empty env var for max_paid_dataset_items becomes None
• Added unit test verifying empty env var for max_total_charge_usd becomes None
• CI passes

🤖 Generated with Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 85.85%. Comparing base (8e0e88d) to head (6e34e2a).
⚠️ Report is 4 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #819      +/-   ##
==========================================
- Coverage   85.88%   85.85%   -0.04%     
==========================================
  Files          46       46              
  Lines        2778     2778              
==========================================
- Hits         2386     2385       -1     
- Misses        392      393       +1     

Flag	Coverage Δ
e2e	37.07% <ø> (ø)
integration	58.60% <ø> (-0.04%)	⬇️
unit	73.39% <ø> (ø)

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