Support multidimensional PostgreSQL arrays

DoltgreSQL supports one-dimensional arrays, but multidimensional array construction and storage are incomplete.

For example, PostgreSQL accepts:

```sql
SELECT ARRAY[ARRAY[1, 2], ARRAY[3, 4]];
SELECT array_to_json(ARRAY[ARRAY[1, 2], ARRAY[3, 4]]);
```

DoltgreSQL currently fails during evaluation with `cannot find cast function from integer[] to integer`. The outer constructor resolves to `integer[]`, then incorrectly attempts to cast each nested `integer[]` child to the scalar `integer` base type. Array serialization also notes that multidimensional values are not currently accounted for.

Acceptance criteria:

- Support nested `ARRAY[...]` constructors with PostgreSQL-compatible type resolution.
- Require matching dimensions for nested arrays and return PostgreSQL-compatible errors for ragged inputs.
- Preserve NULL elements and nested values.
- Round-trip multidimensional arrays through table storage and the PostgreSQL wire protocol.
- Support correct dimension and bound reporting.
- Verify subscripting, slicing, comparison, concatenation, and relevant array functions.
- Enable the skipped multidimensional `array_to_json` integration assertion.
- Validate all regression queries and expected results against a real PostgreSQL server.

Related umbrella issue: #3099

Start with nested ARRAY[...] constructor evaluation and type resolution, then inspect the array serialization path and the skipped multidimensional array_to_json integration assertion. Compare the listed regression queries and expected results with a real PostgreSQL server, including storage and wire-protocol round trips. Done means matching dimensions, NULLs, bounds, subscripting, slicing, comparison, concatenation, and relevant array functions behave compatibly.

Written by the indexing model from the issue text.