[codex] Optimize Build and Test CI runtime

[Widthdom]

Summary

• Limit repeated vulnerability audit and formatting validation in Build and Test CI to the representative ubuntu-latest / net8.0 lane.
• Collect XPlat Code Coverage only on that representative lane while keeping the full OS/framework test matrix.
• Update CI workflow contract tests, docs, and the unreleased changelog fragment for the new CI behavior.

Validation

• dotnet test tests/CodeIndex.Tests/CodeIndex.Tests.csproj --filter "FullyQualifiedName~CiWorkflowTests"
• dotnet run --project tools/CodeIndex.Changelog -- check
• dotnet format CodeIndex.sln --verify-no-changes
• dotnet build
• dotnet ./src/CodeIndex/bin/Debug/net8.0/cdidx.dll status --check --json

Documentation and Changelog

• Updated DEVELOPER_GUIDE.md and TESTING_GUIDE.md.
• Added changelog.d/unreleased/+ci-build-test-runtime.internal.md.

Issues

• No issue auto-close references; this was requested directly.

Follow-up Candidates

• Review CI duration after this lands and decide whether restore/build setup can be split further if the matrix remains slow.