community[patch]: cross_encoders flatten namespaces (#20183)

Issue `langchain_community.cross_encoders` didn't have flattening
namespace code in the __init__.py file.
Changes:
- added code to flattening namespaces (used #20050 as a template)
- added ut for a change
- added missed `test_imports` for `chat_loaders` and
`chat_message_histories` modules

libs/community/tests/unit_tests/chat_loaders/test_imports.py

@@ -0,0 +1,18 @@
+from langchain_community.chat_loaders import _module_lookup
+
+EXPECTED_ALL = [
+    "BaseChatLoader",
+    "FolderFacebookMessengerChatLoader",
+    "GMailLoader",
+    "IMessageChatLoader",
+    "LangSmithDatasetChatLoader",
+    "LangSmithRunChatLoader",
+    "SingleFileFacebookMessengerChatLoader",
+    "SlackChatLoader",
+    "TelegramChatLoader",
+    "WhatsAppChatLoader",
+]
+
+
+def test_all_imports() -> None:
+    assert set(_module_lookup.keys()) == set(EXPECTED_ALL)

libs/community/tests/unit_tests/chat_message_histories/test_imports.py

@@ -0,0 +1,29 @@
+from langchain_community.chat_message_histories import _module_lookup
+
+EXPECTED_ALL = [
+    "AstraDBChatMessageHistory",
+    "CassandraChatMessageHistory",
+    "ChatMessageHistory",
+    "CosmosDBChatMessageHistory",
+    "DynamoDBChatMessageHistory",
+    "ElasticsearchChatMessageHistory",
+    "FileChatMessageHistory",
+    "FirestoreChatMessageHistory",
+    "MomentoChatMessageHistory",
+    "MongoDBChatMessageHistory",
+    "Neo4jChatMessageHistory",
+    "PostgresChatMessageHistory",
+    "RedisChatMessageHistory",
+    "RocksetChatMessageHistory",
+    "SQLChatMessageHistory",
+    "SingleStoreDBChatMessageHistory",
+    "StreamlitChatMessageHistory",
+    "TiDBChatMessageHistory",
+    "UpstashRedisChatMessageHistory",
+    "XataChatMessageHistory",
+    "ZepChatMessageHistory",
+]
+
+
+def test_all_imports() -> None:
+    assert set(_module_lookup.keys()) == set(EXPECTED_ALL)

libs/community/tests/unit_tests/cross_encoders/test_imports.py

@@ -0,0 +1,14 @@
+from langchain_community.cross_encoders import __all__, _module_lookup
+
+EXPECTED_ALL = [
+    "BaseCrossEncoder",
+    "FakeCrossEncoder",
+    "HuggingFaceCrossEncoder",
+    "SagemakerEndpointCrossEncoder",
+]
+
+
+def test_all_imports() -> None:
+    """Test that __all__ is correctly set."""
+    assert set(__all__) == set(EXPECTED_ALL)
+    assert set(__all__) == set(_module_lookup.keys())