fix(dwh): align adapter sampling contract

harness/tht/ports/dwh.py

@@ -21,6 +21,12 @@ class DwhHealth:
     detail: str | None = None
 
 
+@dataclass(frozen=True)
+class DistinctValues:
+    values: list[object]
+    truncated: bool
+
+
 class UnsupportedCapability(Exception):
     """Raised when an adapter cannot provide an optional DWH operation."""
 
@@ -34,10 +40,10 @@ class DwhAdapter(Protocol):
 
     def introspect(self) -> PhysicalSchema: ...
 
-    def run_query(self, sql: str, *, limit: int | None = None) -> ExecResult: ...
+    def run_query(self, sql: str, *, limit: int) -> ExecResult: ...
 
     def explain(self, sql: str) -> PlanSummary: ...
 
     def sample_column(self, table: str, column: str, *, limit: int) -> list[object]: ...
 
-    def distinct_values(self, table: str, column: str) -> list[object]: ...
+    def distinct_values(self, table: str, column: str) -> DistinctValues: ...