fix(dwh): align adapter sampling contract

harness/tests/l0/test_db_sampling.py

@@ -6,7 +6,7 @@ import pytest
 
 from tht.config import LshConfig
 from tht.db.introspect import introspect
-from tht.db.sampling import is_text_type, unique_values_for_lsh
+from tht.db.sampling import distinct_values, is_text_type, sample_column, unique_values_for_lsh
 
 pytestmark = [pytest.mark.l0]
 
@@ -52,3 +52,16 @@ def test_unique_values_for_lsh_truncation_reported(admin_engine):
     truncated_cols = {(t.table, t.column) for t in truncated}
     # fct_ricoveri has several eligible text columns with distinct values
     assert any(t[0] == "fct_ricoveri" for t in truncated_cols)
+
+
+def test_adapter_sampling_is_distinct_and_frequency_ranked(admin_engine):
+    values = sample_column(admin_engine, "dw", "fct_ricoveri", "reparto", limit=2)
+    assert values == ["cardiologia", "pronto soccorso"]
+
+
+def test_adapter_distinct_values_reports_truncation(admin_engine):
+    result = distinct_values(
+        admin_engine, "dw", "fct_ricoveri", "reparto", max_values=1
+    )
+    assert result.values == ["cardiologia"]
+    assert result.truncated is True