Fixes scope of query Session in PGVector (#5194)

`vectorstore.PGVector`: The transactional boundary should be increased
to cover the query itself

Currently, within the `similarity_search_with_score_by_vector` the
transactional boundary (created via the `Session` call) does not include
the select query being made.

This can result in un-intended consequences when interacting with the
PGVector instance methods directly


---------

Co-authored-by: Dev 2049 <dev.dev2049@gmail.com>

langchain/vectorstores/pgvector.py

@@ -298,15 +298,17 @@ class PGVector(VectorStore):
                 for key, value in filter.items():
                     IN = "in"
                     if isinstance(value, dict) and IN in map(str.lower, value):
-                    value_case_insensitive = {k.lower(): v for k, v in value.items()}
+                        value_case_insensitive = {
+                            k.lower(): v for k, v in value.items()
+                        }
                         filter_by_metadata = EmbeddingStore.cmetadata[key].astext.in_(
                             value_case_insensitive[IN]
                         )
                         filter_clauses.append(filter_by_metadata)
                     else:
-                    filter_by_metadata = EmbeddingStore.cmetadata[key].astext == str(
-                        value
-                    )
+                        filter_by_metadata = EmbeddingStore.cmetadata[
+                            key
+                        ].astext == str(value)
                         filter_clauses.append(filter_by_metadata)
 
                 filter_by = sqlalchemy.and_(filter_by, *filter_clauses)
@@ -325,6 +327,7 @@ class PGVector(VectorStore):
                 .limit(k)
                 .all()
             )
+
         docs = [
             (
                 Document(