feat(vectors): support sparse vectors in Milvus

[zhangzhenghao]

Summary

• represent dense and sparse vectors with one Vector type using Values and optional Indices
• update QueryVectors to accept a Vector query
• add shared sparse-vector lifecycle coverage for collection creation, insertion, querying, counting, timestamp deletion, and collection deletion
• implement sparse-vector collections, inserts, and queries for Milvus/Zilliz
• skip the shared sparse test explicitly in backends that do not support it yet

Tests

• TMPDIR=/mnt/openclaw/tmp/gorse-sparse-vector go test ./storage/vectors -count=1
• TMPDIR=/mnt/openclaw/tmp/gorse-sparse-vector go test ./worker ./master -run '^$' -count=1
• go vet ./storage/vectors ./worker ./master
• MILVUS_URI=milvus://127.0.0.1:19530 TMPDIR=/mnt/openclaw/tmp/gorse-sparse-vector go test ./storage/vectors -run '^TestMilvus$' -count=1

The Milvus suite was verified against milvusdb/milvus:v2.6.9.

[codecov]

Codecov Report

❌ Patch coverage is 66.38298% with 79 lines in your changes missing coverage. Please review.
✅ Project coverage is 72.09%. Comparing base (261a018) to head (c65c84e).
⚠️ Report is 2 commits behind head on master.

Files with missing lines	Patch %	Lines
storage/vectors/milvus.go	0.00%	42 Missing ⚠️
storage/vectors/xvec.go	74.56%	26 Missing and 3 partials ⚠️
storage/vectors/qdrant.go	86.27%	4 Missing and 3 partials ⚠️
master/tasks.go	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1362      +/-   ##
==========================================
- Coverage   72.41%   72.09%   -0.33%     
==========================================
  Files          94       94              
  Lines       18415    18608     +193     
==========================================
+ Hits        13336    13416      +80     
- Misses       3693     3771      +78     
- Partials     1386     1421      +35     

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