chore(langgraph): add guide/conformance for delta channel checkpointer (#7736)

## Summary

Add a user-facing design doc and `get_delta_channel_keepset` helper for
third-party `BaseCheckpointSaver` authors who need to support graphs
using `DeltaChannel`.

**Deliverables:**

1. ~~**`docs/delta-channel-checkpointer-guide.md`** — comprehensive
guide covering~~:
moved to docs repo

2. **`BaseCheckpointSaver.get_delta_channel_keepset` /
`aget_delta_channel_keepset`** — returns the minimum set of ancestor
`checkpoint_id`s that must survive deletion for a given head's
`DeltaChannel` reconstruction to remain intact. Enables safe `prune`
implementations without silently corrupting delta history.

3. **Docstring warnings** on `prune`, `aprune`, `delete_for_runs`,
`adelete_for_runs`, `copy_thread`, `acopy_thread` explaining the
DeltaChannel pitfall (silent data loss if ancestor writes/snapshots are
deleted).

4. **Three new conformance capabilities** in
`libs/checkpoint-conformance`:
- `delta_channel_history` — validates the `aget_delta_channel_history`
walk contract
   - `delta_channel_keepset` — validates the keep-set contract
- `delta_channel_reconstruction` — end-to-end round-trip (aput +
aput_writes + history + reconstruct)

## Test plan

- [x] `make format lint` passes in `libs/checkpoint`,
`libs/checkpoint-conformance`
- [x] All three new conformance capabilities pass against
`InMemorySaver`
- [x] Run conformance against SQLite saver 
- [x] Run conformance against Postgres saver

---------

Co-authored-by: Cursor <cursoragent@cursor.com>

libs/checkpoint-sqlite/tests/test_conformance_delta.py

@@ -0,0 +1,37 @@
+"""Run delta-channel conformance capabilities against AsyncSqliteSaver."""
+
+from __future__ import annotations
+
+import pytest
+
+pytest.importorskip(
+    "langgraph.checkpoint.conformance",
+    reason="langgraph-checkpoint-conformance not installed",
+)
+pytest.importorskip("aiosqlite", reason="aiosqlite not installed")
+
+
+@pytest.mark.asyncio
+async def test_delta_channel_conformance():
+    from langgraph.checkpoint.conformance import validate
+    from langgraph.checkpoint.conformance.initializer import checkpointer_test
+
+    from langgraph.checkpoint.sqlite.aio import AsyncSqliteSaver
+
+    @checkpointer_test(name="AsyncSqliteSaver")
+    async def sqlite_saver():
+        async with AsyncSqliteSaver.from_conn_string(":memory:") as saver:
+            yield saver
+
+    report = await validate(
+        sqlite_saver,
+        capabilities={
+            "delta_channel_history",
+            "delta_channel_keepset",
+            "delta_channel_reconstruction",
+        },
+    )
+    for cap, result in report.results.items():
+        if result.passed is False:
+            details = "\n".join(result.failures or [])
+            pytest.fail(f"Capability {cap} failed:\n{details}")