Add cohort_geometries() to access cohort GeoJSON metadata

[adilraza99]

Summary

Add a new cohort_geometries() method to expose cohort boundary geometries through the API.

Changes

• add cohort_geometries() to AnophelesSampleMetadata
• load GeoJSON cohort boundary data using the standard json library
• add caching consistent with existing metadata access patterns
• validate accepted cohort_set values
• add tests and minimal GeoJSON fixtures

Impact

Provides access to cohort geometry metadata while keeping the API simple and dependency-free.

Part of #435

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 90.28%. Comparing base (ab59c87) to head (cb24212).

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1053      +/-   ##
==========================================
+ Coverage   90.25%   90.28%   +0.03%     
==========================================
  Files          51       51              
  Lines        5631     5650      +19     
==========================================
+ Hits         5082     5101      +19     
  Misses        549      549              

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