fix(dwh): align adapter sampling contract

harness/tests/test_dwh_port_contract.py

@@ -1,9 +1,14 @@
+from dataclasses import FrozenInstanceError
+
+import pytest
+
 from tht.execute import ExecResult, PlanSummary
 from tht.mschema.models import PhysicalSchema
 from tht.ports.dwh import (
     DwhAdapter,
     DwhCapabilities,
     DwhHealth,
+    DistinctValues,
     UnsupportedCapability,
 )
 
@@ -17,7 +22,7 @@ class FakeDwhAdapter:
     def introspect(self) -> PhysicalSchema:
         raise NotImplementedError
 
-    def run_query(self, sql: str, *, limit: int | None = None) -> ExecResult:
+    def run_query(self, sql: str, *, limit: int) -> ExecResult:
         raise NotImplementedError
 
     def explain(self, sql: str) -> PlanSummary:
@@ -26,7 +31,7 @@ class FakeDwhAdapter:
     def sample_column(self, table: str, column: str, *, limit: int) -> list[object]:
         raise NotImplementedError
 
-    def distinct_values(self, table: str, column: str) -> list[object]:
+    def distinct_values(self, table: str, column: str) -> DistinctValues:
         raise NotImplementedError
 
 
@@ -45,3 +50,13 @@ def test_contract_types_are_public_and_capabilities_are_immutable():
     assert capabilities.sampling is True
     assert capabilities.distinct_values is True
     assert issubclass(UnsupportedCapability, Exception)
+    with pytest.raises(FrozenInstanceError):
+        capabilities.explain = False
+
+
+def test_all_contract_types_are_exported_from_public_package():
+    from tht.ports import DistinctValues as PublicDistinctValues
+
+    result = PublicDistinctValues(values=["a"], truncated=True)
+    assert result.values == ["a"]
+    assert result.truncated is True