Fix trimmed JSON extraction worker failures

[Widthdom]

Summary

• replace reflection-based JSON property-name serialization in symbol and dependency-lock extraction with trim-safe JsonEncodedText encoding
• replace the large-JSON fallback's reflection-based string deserialization with bounded JsonDocument parsing while preserving malformed-surrogate fallback behavior
• add focused JSON/package-lock coverage plus a published trimmed CLI regression covering version.json, Stryker configuration, package-lock.json, and JSON Schema inputs

Root cause and impact

Published trimmed builds disable reflection-based System.Text.Json serialization. Two extraction-worker code paths serialized runtime property-name strings through JsonSerializer, so valid keys such as $schema and node_modules/... could terminate isolated extraction and leave a rebuild partial. The replacement uses trim-safe primitives and preserves the existing bounded fallback contract for malformed large JSON.

A successful published rebuild now finishes complete and stamps the current index_writer_version; partial runs continue to preserve the previous successful writer stamp as designed.

Validation

• dotnet test CodeIndex.sln -c Release --no-build -p:UseSharedCompilation=false --logger "console;verbosity=quiet" — net8.0: 9,773 passed, 7 skipped; net9.0: 9,306 passed, 413 skipped
• focused Issue v1.39.3: JSON symbol-extraction worker fails with JsonSerializerIsReflectionDisabled, leaving rebuilt index partial #4709 and large-JSON fallback tests on net8.0 and net9.0
• dotnet build CodeIndex.sln -c Release -p:UseSharedCompilation=false
• dotnet build CodeIndex.sln -c Debug -p:UseSharedCompilation=false
• make lint
• dotnet run --project tools/CodeIndex.Changelog --no-restore -- check
• manual macOS arm64 self-contained trimmed publish, rebuild, and status --check --json smoke: complete index, no file errors, writer version 1.39.3
• two-round Codex adversarial review; the first finding was fixed and the second review found no actionable issue

Documentation and changelog

• document the published trimmed JSON worker regression test in both English and Japanese sections of TESTING_GUIDE.md
• add bilingual fragment changelog.d/unreleased/4709.fixed.md
• no README or developer-guide contract change is needed because this restores the already documented trimmed-build behavior

Fixes #4709