Add exact-zero regression coverage for graph count helpers

[Widthdom]

Summary

• add focused exact-zero regression tests for references, callers, and callees
• verify relaxed count helpers respect the caller's --limit in zero-result exact-match flows
• verify exact-zero sample names stay capped at 5 entries
• verify no exact_zero_hint is emitted when the relaxed graph query still returns zero hits
• stamp graph readiness in the test fixture so the graph exact-zero path is exercised directly

Closes #119.

Test plan

• dotnet build
• dotnet test --filter QueryCommandRunnerTests
• dotnet test
• rebuild and refresh the local index with dotnet ./src/CodeIndex/bin/Debug/net8.0/cdidx.dll . --json