release(checkpoint): 4.2.0 (#8563)

Version bump only. No library code changes in this PR.

Unreleased since `checkpoint==4.1.1`:

- #8526 - collect writes at plain-value seed in delta channel history
- #8354 - add opt-in `TTLConfig.omit_expired` to skip expired rows on
read

Minor rather than patch because #8354 adds a new public `TTLConfig` key.
Note that the checkpoint-postgres half of #8354 already shipped in
`checkpointpostgres==3.1.1`, so the currently released Postgres saver
honors an option that no released `langgraph-checkpoint` declares. This
closes that gap.

`langgraph-checkpoint-postgres` 3.1.2 (#8535) follows once this is live
on PyPI.

libs/checkpoint-postgres/uv.lock

@@ -276,7 +276,7 @@ wheels = [
 
 [[package]]
 name = "langgraph-checkpoint"
-version = "4.1.1"
+version = "4.2.0"
 source = { editable = "../checkpoint" }
 dependencies = [
     { name = "langchain-core" },