fix: address review round 3 for migrate-async (#562)

- Pass existing snapshot to create_plan_from_patch to avoid double Redis round-trip
- Use _get_client() instead of _redis_client for lazy async client initialization
- Remap datatype_changes keys to post-rename field names before quantization
- Only resume from completed checkpoint when source index is actually gone

Author: nkanu17

redisvl/migration/async_executor.py

@@ -739,7 +739,8 @@ def _notify(step: str, detail: Optional[str] = None) -> None:
                 effective_changes = datatype_changes
                 if has_field_renames and not resuming_from_checkpoint:
                     field_rename_map = {
-                        fr.old_name: fr.new_name for fr in rename_ops.rename_fields
+                        fr.old_name: fr.new_name
+                        for fr in rename_ops.rename_fields
                     }
                     effective_changes = {
                         field_rename_map.get(k, k): v