estuary-cdk: default tombstone to BaseDocument in open_binding for snapshot bindings

When `fetch_snapshot` is provided but no tombstone is given, `open_binding` now
defaults to `BaseDocument(op="d")` instead of requiring callers to supply one.
This makes it easier to create snapshot bindings without needing to provide a
tombstone explicitly.

Author: Alex-Bair

estuary-cdk/estuary_cdk/capture/common.py

@@ -832,9 +832,14 @@ async def backfill_closure(
             )
 
     if fetch_snapshot:
+        if tombstone is None:
+            # Default tombstone for snapshot bindings so callers don't need to
+            # provide one. The cast satisfies the _BaseDocument TypeVar since
+            # BaseDocument is its bound.
+            tombstone = cast(_BaseDocument, BaseDocument(_meta=BaseDocument.Meta(op="d")))
 
         async def closure(task: Task):
-            assert tombstone
+            assert tombstone is not None
             await _binding_snapshot_task(
                 binding,
                 binding_index,