refactor(dwh): adapt direct and REST transports

harness/tht/rest/execute.py

@@ -46,3 +46,14 @@ def explain_rest(client, sql: str) -> PlanSummary:
     except RestError as e:
         raise ExecutionError(str(e)) from e
     return parse_text_plan(lines)
+
+
+def sample_column_rest(
+    client, schema: str, table: str, column: str, *, limit: int
+) -> list[object]:
+    rows = client.top_values(schema, table, column, limit)
+    return [row["value"] for row in rows if row.get("value") is not None]
+
+
+def distinct_values_rest(client, schema: str, table: str, column: str) -> list[object]:
+    return sample_column_rest(client, schema, table, column, limit=1000)