Add CI flaky-test diagnostics

[Widthdom]

Summary

• Add CI test runsettings with VSTest crash/hang blame capture and uploaded blame artifacts.
• Add a bounded one-time CI retry that marks pass-on-retry runs with flaky-retry.txt.
• Document shared-state parallelism audit rules for tests that touch process-global state.

Validation

• dotnet build CodeIndex.sln -c Release -p:UseSharedCompilation=false
• dotnet test tests/CodeIndex.Tests/CodeIndex.Tests.csproj --filter "FullyQualifiedName~CiWorkflowTests" -p:UseSharedCompilation=false
• dotnet run --project tools/CodeIndex.Changelog -- check
• GIT_CONFIG_COUNT=1 GIT_CONFIG_KEY_0=commit.gpgsign GIT_CONFIG_VALUE_0=false DOTNET_CLI_DO_NOT_USE_MSBUILD_SERVER=1 dotnet test CodeIndex.sln -c Release -p:UseSharedCompilation=false --no-build --no-restore -maxcpucount:1 /nr:false

Documentation and Changelog

• Updated TESTING_GUIDE.md and DEVELOPER_GUIDE.md.
• Added changelog fragments:
  • changelog.d/unreleased/1610.fixed.md
  • changelog.d/unreleased/1612.fixed.md
  • changelog.d/unreleased/1821.fixed.md

Follow-up Candidates

• None.

Fixes #1610
Fixes #1612
Fixes #1821