feat(async search): handle partial response

[forshev]

Description

Add partial response behavior to async search similar to sync search

---

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[github-actions]

🔴 Performance Degradation

Some benchmarks have degraded compared to the previous run.
Click on Show table button to see full list of degraded benchmarks.

Show table

Name	Previous	Current	Ratio	Verdict
Bitmask-4	174405	19fc09
	0.00 B/op	3.00 B/op	NaN	🔴
FindSequence_Random/small-4	174405	19fc09
	10280.15 MB/s	3107.07 MB/s	0.30	🔴
	24.90 ns/op	82.39 ns/op	3.31	🔴
GenerateDocs-4	174405	19fc09
	952.20 ns/op	1099.00 ns/op	1.15	🔴
ProcessDocuments-4	174405	19fc09
	381.00 B/op	32569.00 B/op	85.48	🔴

[codecov-commenter]

Codecov Report

❌ Patch coverage is 20.37037% with 43 lines in your changes missing coverage. Please review.
✅ Project coverage is 71.39%. Comparing base (1744059) to head (4937000).

Files with missing lines	Patch %	Lines
proxyapi/grpc_async_search.go	0.00%	24 Missing ⚠️
proxy/search/async.go	36.66%	13 Missing and 6 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #335      +/-   ##
==========================================
- Coverage   71.65%   71.39%   -0.26%     
==========================================
  Files         204      204              
  Lines       14770    14804      +34     
==========================================
- Hits        10583    10569      -14     
- Misses       3435     3475      +40     
- Partials      752      760       +8     

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