Fix #237 - Support multi-dimensional arrays in DataItemJsonConverter

Refactor WriteFieldValue to handle nested (multi-dimensional) arrays via a
dedicated WriteArrayItems / WriteArrayItemValue pair instead of recursing back
into WriteFieldValue with a sentinel empty-string field name. This:

- Restores GeoJSON support: Polygon/MultiPolygon `coordinates` arrays (depth
  3+) now serialize correctly instead of falling back to
  `System.Collections.Generic.List`1[System.Object]`.
- Removes the implicit `IsNullOrWhiteSpace(propertyName)` sentinel so legitimate
  empty-string and whitespace property names are preserved.
- Adds a MaxJsonDepth (64) guard that throws InvalidOperationException for
  pathological / recursively nested input instead of stack-overflowing the host
  process.

Tests:
- 17 new tests in DataItemJsonConverterTests covering GeoJSON Point/LineString/
  Polygon, the full issue-237 document, mixed scalars/nulls/IDataItems inside
  nested arrays, dictionary-of-dictionary nested arrays, wide-array smoke,
  empty/whitespace property-name regression guards, deserialize/re-serialize
  round-trip, and depth-guard at and over the limit (arrays, objects, mixed).
- All 125 Common.UnitTests pass; full solution test suite is green.

Supersedes #238. Original fix authored by Mart Lankamp - thanks!

Closes #237

Co-authored-by: Mart Lankamp <mart.lankamp@users.noreply.github.com>

Author: philnach