cql2-json: BETWEEN expects the draft nested args shape; final-spec flat args raise TypeError

OGC 21-065r2 (the final CQL2 standard) defines BETWEEN with a flat 3-element `args` array:

```json
{"op": "between", "args": [{"property": "height"}, 2, 5]}
```

`parsers/cql2_json/parser.py` reads the draft nested shape instead (`args[1][0]` / `args[1][1]`):

https://github.com/geopython/pygeofilter/blob/master/pygeofilter/parsers/cql2_json/parser.py#L139-L145

so a spec-conformant client's BETWEEN raises `TypeError: 'int' object is not subscriptable` inside the walker.

Repro:

```python
from pygeofilter.parsers.cql2_json import parse
parse({"op": "between", "args": [{"property": "height"}, 2, 5]})
# TypeError
parse({"op": "between", "args": [{"property": "height"}, [2, 5]]}) # draft shape parses
```

Spec reference: https://docs.ogc.org/is/21-065r2/21-065r2.html (BETWEEN is defined over three scalar expression operands; the JSON encoding's args is flat).

Found while wiring pygeofilter 0.4.0 into an OGC API Features Part 3 endpoint; we currently pre-normalize the flat shape to the nested one before calling `parse`.

Start in parsers/cql2_json/parser.py around lines 139-145 and run the flat-shape reproduction from the issue. Trace how BETWEEN operands are read, then add coverage for the final-spec three-element args form. Done means the flat form parses without TypeError while preserving the existing supported behavior.

Written by the indexing model from the issue text.