Improve robustness of cohorts() metadata access

[adilraza99]

Summary

Improve the robustness of the existing cohorts() method in AnophelesSampleMetadata.

Changes

• add validation for accepted cohort_set values
• add caching to avoid repeated CSV reads
• support optional query parameter for filtering results
• improve error messages for invalid inputs or missing files
• expand test coverage for cohort metadata access

Impact

No changes to the public API.
The update strengthens the reliability and performance of cohorts() while maintaining backward compatibility.

Part of #435

[adilraza99]

Hi @jonbrenas,
Could you please take a look when you have a moment?

[adilraza99]

@jonbrenas

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 90.28%. Comparing base (ab59c87) to head (83408f5).
⚠️ Report is 50 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1052      +/-   ##
==========================================
+ Coverage   90.25%   90.28%   +0.03%     
==========================================
  Files          51       51              
  Lines        5631     5671      +40     
==========================================
+ Hits         5082     5120      +38     
- Misses        549      551       +2     

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