Accelerate SOMA collection mapper graph construction (#597)

* Use tiledb-py to retrieve group members
* Sort imports with ruff
* Add john's comment

src/tiledb/cloud/soma/mapper.py

@@ -4,6 +4,7 @@
 import anndata as ad
 import tiledbsoma
 
+import tiledb
 from tiledb.cloud import dag
 from tiledb.cloud._common import functions
 from tiledb.cloud.utilities import get_logger_wrapper
@@ -233,8 +234,18 @@ def build_collection_mapper_workflow_graph(
             "Retrieving SOMA Experiment URIs from SOMACollection %s"
             % soma_collection_uri
         )
-        with tiledbsoma.Collection.open(soma_collection_uri) as soco:
-            soma_experiment_uris = {k: v.uri for k, v in soco.items()}
+
+        # Alternative:
+        #
+        # with tiledbsoma.Collection.open(soma_collection_uri) as soco:
+        #     soma_experiment_uris = {k: v.uri for k, v in soco.items()}
+        #
+        # -- however, that opens all the members sequentially, and we don't need
+        # that overhead here in the launcher node.
+        #
+        # See also: sc-49443
+        with tiledb.Group(soma_collection_uri) as grp:
+            soma_experiment_uris = {mbr.name: mbr.uri for mbr in grp}
 
     if experiment_names is not None:
         logger.info("Filtering SOMA Experiment URIs for specified names")