Fix trimmed backfill-fold JSON output

[Widthdom]

Summary

• Replace backfill-fold JSON envelopes with source-generated DTOs so the trimmed release no longer aborts on --json.
• Add a regression that publishes a trimmed cdidx.dll and exercises both the success and missing-db JSON paths.
• Update the changelog in both English and Japanese.

Validation

• dotnet build
• dotnet test tests/CodeIndex.Tests/CodeIndex.Tests.csproj --filter FullyQualifiedName~RunBackfillFold_PublishedTrimmedBinary_SerializesSuccessAndErrorJson
• dotnet test tests/CodeIndex.Tests/CodeIndex.Tests.csproj --filter FullyQualifiedName~RunBackfillFold_
• dotnet test tests/CodeIndex.Tests/CodeIndex.Tests.csproj

Documentation / Changelog

• Updated CHANGELOG.md in both language sections.

Fixes #181

Follow-up candidates

• Other CLI JSON paths in QueryCommandRunner still use anonymous-object serialization and may need the same source-generated treatment if trimmed-build JSON support is expanded beyond backfill-fold.

[Widthdom]

Work started on branch fix-issue1074 to address the follow-up candidates from this PR.