fix(dwh): align adapter sampling contract

harness/tht/adapters/dwh/postgres.py

@@ -1,12 +1,14 @@
 """Direct PostgreSQL implementation of the DWH port."""
 
 from tht.config import DatabaseConfig
-from tht.db import execute
+from sqlalchemy.exc import SQLAlchemyError
+
+from tht.db import execute, sampling
 from tht.db.connection import make_engine, ping
 from tht.db.introspect import introspect
 from tht.execute import ExecResult, PlanSummary
 from tht.mschema.models import PhysicalSchema
-from tht.ports.dwh import DwhCapabilities, DwhHealth
+from tht.ports.dwh import DistinctValues, DwhCapabilities, DwhHealth
 
 
 class PostgresDwhAdapter:
@@ -19,23 +21,25 @@ class PostgresDwhAdapter:
     def health(self) -> DwhHealth:
         try:
             ping(self._engine)
-        except Exception as exc:
+        except SQLAlchemyError as exc:
             return DwhHealth(ok=False, detail=str(exc))
         return DwhHealth(ok=True)
 
     def introspect(self) -> PhysicalSchema:
         return introspect(self._engine, self._config.database, self._config.db_schema)
 
-    def run_query(self, sql: str, *, limit: int | None = None) -> ExecResult:
+    def run_query(self, sql: str, *, limit: int) -> ExecResult:
         return execute.run_query(self._engine, sql, limit=limit)
 
     def explain(self, sql: str) -> PlanSummary:
         return execute.explain(self._engine, sql)
 
     def sample_column(self, table: str, column: str, *, limit: int) -> list[object]:
-        return execute.sample_column(
+        return sampling.sample_column(
             self._engine, self._config.db_schema, table, column, limit=limit
         )
 
-    def distinct_values(self, table: str, column: str) -> list[object]:
-        return execute.distinct_values(self._engine, self._config.db_schema, table, column)
+    def distinct_values(self, table: str, column: str) -> DistinctValues:
+        return sampling.distinct_values(
+            self._engine, self._config.db_schema, table, column
+        )