[Test] Added ensemble posterior weights validation

[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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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 78.66%. Comparing base (3ff1f51) to head (a9e2775).

Additional details and impacted files

@@             Coverage Diff             @@
##             main    #1307       +/-   ##
===========================================
- Coverage   89.35%   78.66%   -10.70%     
===========================================
  Files         118      118               
  Lines        8709     8703        -6     
===========================================
- Hits         7782     6846      -936     
- Misses        927     1857      +930     

Flag	Coverage Δ
unittests	78.66% <ø> (-10.70%)	⬇️

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see 33 files with indirect coverage changes

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