enhance: validate SQN resource group capacity

[sunby]

Summary

• Add SQN-aware replica admission checks in AssignReplica when streaming and EnableSQNServeSegments are enabled.
• Enforce total SQN capacity plus strict and non-strict resource group isolation semantics, including flat fallback when the default SQN pool is absent.
• Add unit coverage, a PyMilvus namespace create/insert/index/load/query flow, and narrow scripts/stop.sh to Milvus run processes only.

Test Plan

• PKG_CONFIG_PATH="$PWD/internal/core/output/lib/pkgconfig:$PWD/cmake_build/src:${PKG_CONFIG_PATH:-}" CGO_LDFLAGS="-Wl,-rpath,$PWD/internal/core/output/lib" CGO_LDFLAGS_ALLOW="-Wl,-rpath,.*" go test -tags dynamic,test -gcflags="all=-N -l" -count=1 ./internal/querycoordv2/utils
• make build-go
• PYTHONPATH=/Users/sunbingyi-zilliz/pymilvus /Users/sunbingyi-zilliz/milvus/tests/python_client/.venv/bin/python -m pytest /Users/sunbingyi-zilliz/milvus/tests/python_client/testcases/test_pymilvus_basic_flow.py -q against local SN-only cluster
• CLANG_FORMAT="$(brew --prefix llvm@15)/bin/clang-format" internal/core/run_clang_format.sh internal/core

Notes

• This is stacked on sunby/milvus:codex/load-1m-segments-pr-stack because origin/master does not contain QueryCoordCfg.EnableSQNServeSegments.
• make lint-fix was attempted and still fails on the base stack typecheck issue: internal/metastore/kv/querycoord/kv_catalog_test.go:372:15: undefined: mocks.