Fix value-level greater_than comparator (used < instead of >)

[Rdag15]

ValueLevelMetadata.greater_than() returns value < check_value — an exact copy of less_than() — so Define-XML value-level where-clauses with Comparator="GT" select the inverse set of rows. greater_than_or_equal_to() already uses >=, so the intended direction is clear.

Repro on main:

from unittest.mock import Mock
import pandas
from odmlib.define_2_1.model import CheckValue
from cdisc_rules_engine.models.define import ValueLevelMetadata

vlm = ValueLevelMetadata()
vlm.item = Mock(Name="TEST")
vlm.check_values = [CheckValue(_content="2")]
df = pandas.DataFrame.from_dict({"TEST": [3]})
print(vlm.greater_than()(df.iloc[0]))   # 3 > 2 -> should be True; on main prints False

Fix: change < to > in greater_than(). Added a test covering values above, at, and below the check value. One-line change plus test; no other comparator is affected.