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
• make format && make lint clean in libs/checkpoint and libs/langgraph