fix: read indices_test.pkl in get_shadow_model

[shamykyzer]

Fixes #442

get_shadow_model in sacroml/attacks/utils.py was opening indices_train.pkl twice, so the returned indices_test actually contained the train indices.

• save_shadow_model has always written both files correctly (since feat: save and reuse trained shadow models #357)

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.51%. Comparing base (42041fd) to head (4054a31).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #443   +/-   ##
=======================================
  Coverage   99.51%   99.51%           
=======================================
  Files          23       23           
  Lines        2687     2687           
=======================================
  Hits         2674     2674           
  Misses         13       13           

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