[test] add tests for ensemble posterior weights

[samadpls]

What does this implement/fix? Explain your changes

Adds a test case for weight validation in EnsemblePosterior to verify proper handling of different weight formats (list, tensor) and error cases. The test ensures the API correctly validates weights without running full posterior accuracy checks.

Does this close any currently open issues?

Fixes #1304

Checklist

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• I have read and understood the contribution
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  with pytest.mark.slow.
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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 78.62%. Comparing base (3ff1f51) to head (19ceaba).
Report is 1 commits behind head on main.

Additional details and impacted files

@@             Coverage Diff             @@
##             main    #1307       +/-   ##
===========================================
- Coverage   89.35%   78.62%   -10.74%     
===========================================
  Files         118      118               
  Lines        8709     8716        +7     
===========================================
- Hits         7782     6853      -929     
- Misses        927     1863      +936     

Flag	Coverage Δ
unittests	78.62% <ø> (-10.74%)	⬇️

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see 32 files with indirect coverage changes

[janfb]

Looks good overall, thanks! 🙏

I have one suggestion, see comment below.

[janfb]

Looks good! Thanks a lot for adding this 🙏