fix: Serialize native LIST/STRUCT/ARRAY/UNION columns per-row in REST JSON (#89)

[jrosskopf]

Summary

Fixes #89 — the REST JSON serializer corrupted native nested-type columns:

• LIST(VARCHAR) — each row reported the concatenation of every row's list in the chunk (e.g. 4 rows × 3 elements → a 12-element array per row).
• LIST(STRUCT) — correct length, but every element was a copy of element[0] (the original report).
• STRUCT — multi-row results always read element[0].
• ARRAY (found in review) — routed through the LIST path, which after the fix reads duckdb_list_entry metadata that fixed-size arrays don't have.
• UNION (found in review) — routed through the struct path, which emitted the tag + every candidate member instead of only the active one.

to_json(col) was unaffected (JSON-typed column path), which matched the reporter's workaround and pointed straight at per-row child-vector handling.

Root cause

• convertVectorListToJson iterated the entire child vector (duckdb_list_vector_get_size) instead of the row's own duckdb_list_entry {offset, length} slice.
• convertVectorStructToJson always passed child index 0 instead of row_idx.
• ARRAY/UNION were aliased onto the list/struct paths, which is incorrect for their physical layouts.

Fix

• List: read the per-row duckdb_list_entry offset/length and only emit that slice.
• Struct: pass row_idx through to each child vector.
• Array: new convertVectorArrayToJson using duckdb_array_type_array_size + duckdb_array_vector_get_child (child_idx = row_idx * array_size + i).
• Union: new convertVectorUnionToJson — read the tag (struct child 0, uint8), resolve the active member (struct child tag+1), emit it as {member_name: value} to match DuckDB's own to_json.
• All paths now honor row validity, so NULL nested values serialize as null.

Test plan

• New [query_executor][list] regression tests: single/multi-row LIST(STRUCT), multi-row LIST(VARCHAR), multi-row STRUCT, fixed-size ARRAY, multi-row UNION, and NULL list entries (red before fix, green after).
• Full C++ unit suite: 643/643 passing.
• Reviewed with codex (surfaced the ARRAY + UNION regressions, both fixed and re-reviewed LGTM).

Closes #89