[steveisok]

Layer 2 of the native decomposition for dotnet#5981. This PR is based on and depends on dotnet#5998.

This layer stabilizes the reusable SOS harness across hosts and architectures: bounded child-process capture and stream drainage, clearer child-host failures, bounded LLDB dump hosts, second-chance DbgEng crash handling, macOS sos-lldb integration, RID-aware debuggee builds, and focused platform/configuration validity checks.

It intentionally excludes Helix submission and pipeline wiring, payload overlays, sharding, command-coverage migration, and legacy test deletion.

Validation:

• dotnet build src/tests/SOS.Tests/SOS.Tests.csproj -c Debug -p:TargetArch=arm64 -p:TargetRid=osx-arm64 --no-restore
• focused BoundedProcessTests, HostSlotTests, and TestConfigValidityTests (20 passed)
• dotnet publish for the SosHarnessScenarios single-file osx-arm64 debuggee
• ./build.sh -skipmanaged -configuration Debug -architecture arm64

[steveisok]

Superseded by upstream-owned stack layer dotnet#6000, which contains the same tested commit and participates in native stack dotnet/diagnostics#6003.