…ist (#7582)

## Summary

Fixes #7498 — `MESSAGE_COERCION_FAILURE` when resuming threads
checkpointed before v1.0.1.

**Root cause:** PR #6269 (v1.0.1) added an `_allowed_json_modules`
security gate to `JsonPlusSerializer._reviver`. The gate defaults to
`None`, so old `"json"`-format checkpoint blobs containing `lc=2`
constructor dicts (the pre-msgpack serialization format for pydantic
objects like `HumanMessage`) are now returned as raw dicts instead of
being reconstructed. Those raw dicts reach `add_messages →
convert_to_messages`, which sees `type="constructor"` and raises
`MESSAGE_COERCION_FAILURE`. Fresh first-turn messages are unaffected
because current `dumps_typed` only writes `"msgpack"` blobs.

**Fix:** `_reviver` now reconstructs `lc=2` blobs whose target class is
already in `SAFE_MSGPACK_TYPES` — the same curated allowlist already
used by the msgpack deserialization path (includes all standard
LangChain message types). Unknown classes are still blocked, preserving
the security intent of #6269.

## Changes

- `libs/checkpoint/langgraph/checkpoint/serde/jsonplus.py` — add
`_is_safe_json_type()` helper; update `_reviver` and
`_check_allowed_json_modules` to allow safe types without an explicit
allowlist
- `libs/checkpoint/tests/test_jsonplus.py` — two new regression tests:
safe-type `lc=2` blobs revive correctly; unknown-type `lc=2` blobs stay
blocked

## Test plan

- [ ] `test_lc2_json_safe_type_revives_without_allowlist` —
`HumanMessage`/`AIMessage` lc=2 JSON blobs round-trip to proper
`BaseMessage` objects with no allowlist configured
- [ ] `test_lc2_json_unknown_type_stays_blocked_without_allowlist` —
`pprint.pprint` lc=2 blob still returns raw dict (not reconstructed)
- [ ] `test_deserde_invalid_module` — existing behaviour unchanged
- [ ] Full `test_jsonplus.py` suite: 93/93 passing

Co-authored-by: Claude Sonnet 4.6 <noreply@anthropic.com>

- Rm untyped loads/dumps
- Restrict to an allow list

---------

Co-authored-by: Sydney Runkle <sydneymarierunkle@gmail.com>