Add unit test for .psis_subset() output structure

[utkarshpawade]

Fixes #518

Summary

• Adds a test verifying .psis_subset() preserves psis object structure (class, log_weights, diagnostics, dims, norm_const_log, tail_len, r_eff)
• Acts as a canary if loo changes its psis internals

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.67%. Comparing base (94a2ac5) to head (7c10083).

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #519   +/-   ##
=======================================
  Coverage   98.67%   98.67%           
=======================================
  Files          35       35           
  Lines        5903     5903           
=======================================
  Hits         5825     5825           
  Misses         78       78           

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