docs(checkpoint): mark DeltaChannel and delta-history APIs as beta (#7732)

## Summary

Adds a Beta admonition to the `DeltaChannel` surface area so users can
distinguish stable from in-progress contracts. Single-sourced on the
base class; subclass overrides in `checkpoint-postgres` /
`checkpoint-sqlite` / memory inherit the marker via their existing
references back to the base.

Touched:
- `DeltaChannel` (`libs/langgraph/langgraph/channels/delta.py`)
- `BaseCheckpointSaver.get_delta_channel_history` and
`aget_delta_channel_history` (`libs/checkpoint/.../base/__init__.py`)
- `DeltaChannelHistory` TypedDict
- `CheckpointMetadata.delta_updates_since_snapshot`

## Why docstring admonitions, not `@beta`

The base `get_delta_channel_history` methods are designed to be
overridden by savers. Wrapping with `@beta` (from `langchain_core._api`)
would emit warnings whenever a subclass called
`super().get_delta_channel_history(...)` or whenever the default
ancestor walk fired. Docstring-only keeps the signal advisory and
noise-free. `_DeltaSnapshot` is already leading-underscore-private, so
it implicitly signals "internal."

## Versioning note

This intentionally stays a **minor** bump for the checkpoint releases
(4.0 → 4.1, 3.0 → 3.1):

- The new methods are strictly additive — defaults provided on the base,
no signatures changed, no removed APIs. Third-party savers keep working
without overriding anything.
- The beta marker and the version bump do orthogonal jobs: semver
answers "is this a breaking change?" (no), the marker answers "is this
contract stable?" (no).
- Bumping major now would consume the lever you want available for when
the delta contract actually changes in a breaking way.

## Test plan

- [ ] Docstring-only — no behavioral change
- [x] `make format && make lint` clean in `libs/checkpoint` and
`libs/langgraph`

libs/checkpoint/langgraph/checkpoint/base/__init__.py

@@ -63,6 +63,11 @@ class CheckpointMetadata(TypedDict, total=False):
     delta_updates_since_snapshot: dict[str, int]
     """Per-channel update count since the last `_DeltaSnapshot` was written.
 
+    !!! warning "Beta"
+
+        This metadata field backs `DeltaChannel` (beta). The key name and
+        contents may change while the delta-channel design stabilizes.
+
     Maps channel name → number of supersteps that wrote to this channel
     since its last snapshot blob. Used by `pregel.create_checkpoint` to
     decide when to write the next snapshot (when the count reaches the
@@ -135,6 +140,11 @@ class CheckpointTuple(NamedTuple):
 class DeltaChannelHistory(TypedDict):
     """Per-channel result entry from `BaseCheckpointSaver.get_delta_channel_history`.
 
+    !!! warning "Beta"
+
+        Part of the `DeltaChannel` support surface; in beta. Field names and
+        semantics may change.
+
     Storage-level view of what one channel contributed across the ancestor
     chain of a target checkpoint:
 
@@ -497,6 +507,14 @@ class BaseCheckpointSaver(Generic[V]):
     ) -> Mapping[str, DeltaChannelHistory]:
         """Walk the parent chain returning per-channel writes + seed.
 
+        !!! warning "Beta"
+
+            This method is part of the `DeltaChannel` support surface and is
+            in beta. The signature, return shape (`DeltaChannelHistory`), and
+            interaction with `_DeltaSnapshot` blobs may change. Override at
+            your own risk; the default implementation will continue to work
+            against the public `BaseCheckpointSaver` contract.
+
         For each requested channel, walks ancestors of the checkpoint
         identified by `config` (following `parent_config`) and accumulates
         `pending_writes` for that channel. The walk terminates per-channel
@@ -556,7 +574,13 @@ class BaseCheckpointSaver(Generic[V]):
     async def aget_delta_channel_history(
         self, *, config: RunnableConfig, channels: Sequence[str]
     ) -> Mapping[str, DeltaChannelHistory]:
-        """Async version of `get_delta_channel_history`."""
+        """Async version of `get_delta_channel_history`.
+
+        !!! warning "Beta"
+
+            This method is part of the `DeltaChannel` support surface and is
+            in beta. See `get_delta_channel_history` for caveats.
+        """
         if not channels:
             return {}
         collected_by_ch: dict[str, list[PendingWrite]] = {c: [] for c in channels}

libs/langgraph/langgraph/channels/delta.py

@@ -26,6 +26,15 @@ class DeltaChannel(Generic[Value], BaseChannel[Any, Any, Any]):
     """Reducer channel that stores only a sentinel in checkpoint blobs and
     reconstructs state by replaying ancestor writes through the reducer.
 
+    !!! warning "Beta"
+
+        `DeltaChannel` is in beta. The API and on-disk representation may
+        change in future releases. Threads written with `DeltaChannel` today
+        are expected to remain readable, but the surrounding contract
+        (`BaseCheckpointSaver.get_delta_channel_history`, the
+        `_DeltaSnapshot` blob shape, the `delta_updates_since_snapshot`
+        metadata field) is not yet stable.
+
     The reducer receives the current accumulated value and a batch of writes
     in one call: `reducer(state, [write1, write2, ...]) -> new_state`.