fix: rename channels._delta → channels.delta throughout

Replaces all `channels._delta` import paths with the new public
`channels.delta` module added in this branch.

Co-Authored-By: Claude Sonnet 4.6 (1M context) <noreply@anthropic.com>

libs/checkpoint-postgres/tests/test_async.py

@@ -377,13 +377,13 @@ async def test_get_checkpoint_no_channel_values(
 async def test_delta_channel_chain_reconstruction(saver_name: str) -> None:
     """AsyncPostgresSaver reconstructs DeltaChannel chain via point-lookup traversal."""
     pytest.importorskip(
-        "langgraph.channels._delta", reason="langgraph core not installed"
+        "langgraph.channels.delta", reason="langgraph core not installed"
     )
 
     from typing import Annotated
 
     from langchain_core.messages import AIMessage, HumanMessage
-    from langgraph.channels._delta import DeltaChannel
+    from langgraph.channels.delta import DeltaChannel
     from langgraph.graph import START, StateGraph
     from langgraph.graph.message import add_messages
     from typing_extensions import TypedDict

libs/langgraph/langgraph/graph/state.py

@@ -47,9 +47,9 @@ from langgraph._internal._fields import (
 from langgraph._internal._pydantic import create_model
 from langgraph._internal._runnable import coerce_to_runnable
 from langgraph._internal._typing import EMPTY_SEQ, MISSING, DeprecatedKwargs
-from langgraph.channels._delta import DeltaChannel
 from langgraph.channels.base import BaseChannel
 from langgraph.channels.binop import BinaryOperatorAggregate, _strip_extras
+from langgraph.channels.delta import DeltaChannel
 from langgraph.channels.ephemeral_value import EphemeralValue
 from langgraph.channels.last_value import LastValue, LastValueAfterFinish
 from langgraph.channels.named_barrier_value import (

libs/langgraph/tests/test_delta_channel_migration.py

@@ -49,7 +49,6 @@ import pytest
 from langgraph.checkpoint.memory import InMemorySaver
 from typing_extensions import TypedDict
 
-from langgraph.channels._delta import DeltaChannel
 from langgraph.channels.binop import BinaryOperatorAggregate
 from langgraph.channels.delta import DeltaChannel
 from langgraph.graph import END, START, StateGraph

libs/langgraph/tests/test_pregel.py

@@ -9408,7 +9408,6 @@ async def test_delta_channel_end_to_end_inmemory() -> None:
     from langchain_core.messages import AIMessage, HumanMessage
     from langgraph.checkpoint.memory import InMemorySaver
 
-    from langgraph.channels._delta import DeltaChannel
     from langgraph.graph import START, StateGraph
     from langgraph.graph.message import add_messages
 
@@ -9450,7 +9449,6 @@ async def test_delta_channel_time_travel() -> None:
     from langchain_core.messages import AIMessage, HumanMessage
     from langgraph.checkpoint.memory import InMemorySaver
 
-    from langgraph.channels._delta import DeltaChannel
     from langgraph.graph import START, StateGraph
     from langgraph.graph.message import add_messages
 
@@ -9508,7 +9506,6 @@ async def test_delta_channel_remove_message_end_to_end() -> None:
     from langchain_core.messages import AIMessage, HumanMessage, RemoveMessage
     from langgraph.checkpoint.memory import InMemorySaver
 
-    from langgraph.channels._delta import DeltaChannel
     from langgraph.graph import START, StateGraph
     from langgraph.graph.message import add_messages
 
@@ -9555,7 +9552,6 @@ async def test_delta_channel_update_by_id_end_to_end() -> None:
     from langchain_core.messages import HumanMessage
     from langgraph.checkpoint.memory import InMemorySaver
 
-    from langgraph.channels._delta import DeltaChannel
     from langgraph.graph import START, StateGraph
     from langgraph.graph.message import add_messages