feat(storage): support purging vector databases

[zhangzhenghao]

Summary

• add Purge to the vector database contract
• purge vector databases by deleting every collection for Xvec, Qdrant, Milvus, and Weaviate
• match data/cache behavior for unavailable and proxy databases
• add shared purge coverage, including repeated purge of an empty database

Tests

• go test ./storage/vectors -count=1
• go test -p 1 ./... -run '^$'
• git diff --check

[codecov]

Codecov Report

❌ Patch coverage is 40.54054% with 22 lines in your changes missing coverage. Please review.
✅ Project coverage is 72.24%. Comparing base (725fe38) to head (c1d090d).

Files with missing lines	Patch %	Lines
storage/vectors/milvus.go	0.00%	16 Missing ⚠️
storage/vectors/database.go	55.55%	2 Missing and 2 partials ⚠️
storage/vectors/xvec.go	60.00%	1 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1365      +/-   ##
==========================================
+ Coverage   71.97%   72.24%   +0.27%     
==========================================
  Files          94       94              
  Lines       18838    18874      +36     
==========================================
+ Hits        13558    13636      +78     
+ Misses       3846     3832      -14     
+ Partials     1434     1406      -28     

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