Increase balance package test coverage (#281)

Summary:

The coverage report showed 98% overall coverage with 7 files having gaps. These tests cover previously untested edge cases including:

- CLI exception handling paths for weighting failures
- Sample class design effect diagnostics and IPW model parameters
- CBPS optimization convergence warnings and constraint violation exceptions
- Plotting functions with missing values, default parameters, and various dist_types
- Distance metrics with empty numeric columns

Differential Revision: D90946146

Author: talgalili