Fix CohereReranker bug

[svilupp]

• Fixed a bug in CohereReranker when it wouldn't handle correctly CandidateChunks.

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 3 lines in your changes missing coverage. Please review.

Project coverage is 91.80%. Comparing base (b16dd25) to head (976bb20).

Files	Patch %	Lines
src/Experimental/RAGTools/retrieval.jl	0.00%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #176      +/-   ##
==========================================
- Coverage   91.83%   91.80%   -0.03%     
==========================================
  Files          45       45              
  Lines        3759     3760       +1     
==========================================
  Hits         3452     3452              
- Misses        307      308       +1     

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