fix dict utils

[arrjon]

The change corrects a conditional statement to ensure proper validation of the length of variable_names against the shape of x. The previous statement only worked for integer smaller than 255 as is not checks for object identity not the values.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Files with missing lines	Coverage Δ
bayesflow/utils/dict_utils.py	62.12% <100.00%> (ø)