Fix empty compact callees results

[Widthdom]

Summary

• preserve the empty compact location envelope for zero-match callees queries
• keep bounded output with and without explicit --fields aligned with references and callers
• add regression coverage for zero result and authoritative cardinality metadata

Validation

• dotnet restore CodeIndex.sln
• dotnet build CodeIndex.sln -c Release -p:UseSharedCompilation=false --no-restore
• dotnet test tests/CodeIndex.Tests/CodeIndex.Tests.csproj -c Release -p:UseSharedCompilation=false --filter "FullyQualifiedName~JsonEnvelopeWrapperIssue4585Tests.BoundedProjection_AllRequestedCommandFamiliesExposeSharedMetadata_Issue4585"
• dotnet test CodeIndex.sln -c Release -p:UseSharedCompilation=false --no-build --no-restore (net8.0: 11,756 total, 0 failed; net9.0: 11,693 total, 0 failed)
• dotnet format CodeIndex.sln --verify-no-changes --no-restore
• dotnet run --project tools/CodeIndex.Changelog -- check
• CLI smoke checks for the reported command, both with and without --fields path,line
• codex review --base origin/main: no blocking/actionable issues found

Documentation and changelog

• updated both English and Japanese sections of TESTING_GUIDE.md
• added bilingual changelog.d/unreleased/5128.fixed.md

Follow-up candidates

• None.

Fixes #5128