feat(balancer): add IPv6-only endpoint filtering

[kprokopenko]

Pull request type

• Feature
• Bugfix
• Code style update (formatting, renaming)
• Refactoring (no functional changes, no api changes)
• Build related changes
• Documentation content changes
• Other (please describe):

What is the current behavior?

Discovery may return FQDN endpoints without ip_v6 values. In this case gRPC resolves the name itself and can establish a connection over IPv4.

Issue Number: N/A

What is the new behavior?

• Added experimental balancers.OnlyIPVersion(..., balancers.IPv6).
• FQDN endpoints are resolved to IPv6 during Discovery.
• Endpoints without IPv6 addresses are excluded.

Other information

Validated with:

• go test ./balancers ./internal/discovery ./internal/balancer
• go test -race -tags integration ./tests/integration -run '^TestOnlyIPVersion$' -count=1
• golangci-lint run ./...

[github-actions]

github.com/ydb-platform/ydb-go-sdk/v3/balancers

compatible changes

IPVersion: added
IPv6: added
OnlyIPVersion: added

summary

Inferred base version: v3.143.0
Suggested version: v3.144.0

[codecov]

Codecov Report

❌ Patch coverage is 85.20710% with 25 lines in your changes missing coverage. Please review.
✅ Project coverage is 72.25%. Comparing base (541e672) to head (ff612fc).

Files with missing lines	Patch %	Lines
internal/conn/pool.go	7.14%	13 Missing ⚠️
internal/balancer/config/routerconfig.go	84.00%	4 Missing ⚠️
internal/conn/conn.go	55.55%	3 Missing and 1 partial ⚠️
internal/balancer/balancer.go	88.23%	1 Missing and 1 partial ⚠️
internal/xresolver/xresolver.go	94.59%	1 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2239      +/-   ##
==========================================
- Coverage   75.27%   72.25%   -3.02%     
==========================================
  Files         443      443              
  Lines       45910    46015     +105     
==========================================
- Hits        34557    33249    -1308     
- Misses       9401    10745    +1344     
- Partials     1952     2021      +69     

Flag	Coverage Δ
experiment	?
go-1.21.x	72.18% <85.20%> (+0.03%)	⬆️
go-1.26.x	72.18% <85.20%> (-3.08%)	⬇️
integration	56.84% <70.41%> (-0.01%)	⬇️
macOS	45.30% <66.86%> (+0.16%)	⬆️
ubuntu	72.25% <85.20%> (-3.02%)	⬇️
unit	45.31% <66.86%> (+0.15%)	⬆️
windows	45.31% <66.86%> (+0.16%)	⬆️
ydb-24.4	56.40% <70.41%> (+0.04%)	⬆️
ydb-edge	56.64% <70.41%> (-0.03%)	⬇️
ydb-latest	56.53% <70.41%> (-0.11%)	⬇️
ydb-nightly	?

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