Fix discovery JSON shape consistency

[Widthdom]

Summary

• Unify symbols and files discovery row serialization across NDJSON, arrays, and envelopes.
• Keep projected rows in results while preserving exact-readiness and truthful pagination metadata, including empty and summary-only cases.
• Add .NET 8/.NET 9 contract coverage for projected and unprojected output shapes.

Root cause

The bounded envelope wrapper treated any first discovery JSON object as a nested payload, relocating a flat row into response context. Raw symbol arrays also bypassed the discovery-row builder that adds exact readiness.

Documentation and changelog

• Updated the English and Japanese output-contract sections in DEVELOPER_GUIDE.md.
• Added changelog.d/unreleased/4835.fixed.md.

Validation

• dotnet restore CodeIndex.sln
• dotnet build CodeIndex.sln -c Release -p:UseSharedCompilation=false
• dotnet test CodeIndex.sln -c Release -p:UseSharedCompilation=false --no-build (10,517 passed; 7 skipped; 0 failed)
• dotnet test tests/CodeIndex.Tests/CodeIndex.Tests.csproj -c Release --no-build --no-restore --filter "FullyQualifiedName~JsonEnvelopeWrapper|FullyQualifiedName~QueryCommandRunnerIssue4835Tests" -p:UseSharedCompilation=false (41 passed on each of .NET 8 and .NET 9)
• dotnet run --project tools/CodeIndex.Changelog -- check
• Codex adversarial review (two initial findings addressed; final review found no actionable issues)

Follow-up candidates

None.

Fixes #4835