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 99.15%. Comparing base (44c3e5c) to head (6734368).

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #519   +/-   ##
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  Coverage   99.15%   99.15%           
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  Files          35       35           
  Lines        5920     5920           
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  Hits         5870     5870           
  Misses         50       50           

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[Copilot]

Pull request overview

Adds a regression/unit test to ensure .psis_subset() returns a valid loo::psis object with expected internal structure, acting as a canary if loo changes psis internals.

Changes:

• Added a new test verifying .psis_subset() output class, log_weights, diagnostics, and key attributes/dimensions.
• Added a NEWS entry noting the new unit test.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 3 comments.

File	Description
tests/testthat/test-ppc-loo.R	Adds a structure-focused unit test for .psis_subset() output.
NEWS.md	Documents the addition of the new unit test.

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[jgabry]

I don't think we need this because we already have ppc_loo_intervals and ppc_loo_ribbon tests that use the subset argument. Those tests would fail if loo changes something that breaks .psis_subset.