Fix JSON array symbol hierarchy

[Widthdom]

Summary

• Normalize JSON and JSON Lines array paths so array indices attach directly while literal object keys remain dot-qualified.
• Preserve root arrays, nested arrays and objects, empty containers, comments, compact one-line inputs, and legal empty/NUL-containing keys with accurate line and parent metadata.
• Keep JSON/JSONL outline paths parent-first with correct depth and pagination, without changing JavaScript bracketed-property behavior.
• Bump the JSON and JSON Lines extractor contracts so existing indexes are rebuilt with the corrected hierarchy.

Root cause and impact

Array indices and object-property boundaries shared path-composition logic, which could duplicate parent names or conflate literal bracket-prefixed keys with array elements. Line lookup also depended on raw-text matching, so comments, bounded skips, compact files, and empty property names could misalign later symbols. As a result, symbols, definitions, and outlines could expose incorrect paths, parents, depths, or signatures for valid JSON/JSONL inputs.

The updated extraction and outline contracts now preserve an unambiguous hierarchy across those cases.

Validation

• dotnet build CodeIndex.sln -c Release --no-restore -p:UseSharedCompilation=false — succeeded with 0 warnings and 0 errors.
• Issue Fix duplicated JSON array-object paths and flattened depth #4874 focused tests on net8.0 and net9.0 — 10 passed on each framework.
• Broader JSON extraction tests on net8.0 — 24 passed.
• Broader outline tests on net8.0 — 50 passed.
• Full Release net8.0 suite — 10,809 passed, 7 skipped, 0 failed.
• Full Release net9.0 suite — 10,334 passed, 420 skipped, with one unrelated filesystem-watcher test failure already tracked by Use one membership and invalidation policy for scan, status, and watch #4592; the same test failed in isolation under concurrent load, while its net8.0 full-suite run passed.
• dotnet format CodeIndex.sln --no-restore --verify-no-changes — passed.
• Changelog fragment checker — all 48 fragments passed.
• git diff --check — passed.
• Codex adversarial review — no blocking/actionable issues found.

Documentation and changelog

• Updated the English and Japanese JSON/JSON Lines sections in DEVELOPER_GUIDE.md.
• Added changelog.d/unreleased/4874.fixed.md.

Follow-up candidates

• None.

Fixes #4874