fix: make computed columns explicitly local-only

Both operations were advertised on remote tables and neither could work. A
declaration reaches the wire as AllNulls, which RemoteTable::add_columns does
not accept, and refresh_column fell through to the BaseTable default; the
TypeScript wrapper reached the same surface. Callers got errors that named
neither the feature nor the reason.

The remote protocol has no representation for a stored expression, and what
one should look like is not settled -- the server persists a declaration under
a different vocabulary. So this states the boundary rather than guessing at a
wire format: an explicit AllNulls arm, a default that names computed columns,
and NotImplementedError raised in Python before the round trip.

python/python/lancedb/remote/table.py

@@ -964,10 +964,14 @@ class RemoteTable(Table):
         *,
         computed: Dict[str, str] | None = None,
     ) -> AddColumnsResult:
-        return LOOP.run(self._table.add_columns(transforms, computed=computed))
+        if computed:
+            raise NotImplementedError(
+                "computed columns are supported only on local tables"
+            )
+        return LOOP.run(self._table.add_columns(transforms))
 
     def refresh_column(self, column: str):
-        return LOOP.run(self._table.refresh_column(column))
+        raise NotImplementedError("computed columns are supported only on local tables")
 
     def alter_columns(
         self, *alterations: Iterable[Dict[str, str]]