…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>

## Summary

Ports Python time-travel fixes
([#7038](langchain-ai/langgraph#7038),
[#7115](langchain-ai/langgraph#7115),
[#7498](langchain-ai/langgraph#7498),
[#7499](langchain-ai/langgraph#7499)) into
`@langchain/langgraph` so replay/fork behave correctly with interrupts
and nested subgraphs.

- **Stale `RESUME` on replay** — Replaying from a checkpoint before an
interrupt no longer consumes cached resume writes; interrupts re-fire
with the correct payload.
- **Subgraph checkpoint loading on time travel** — Introduces
`ReplayState` (`CONFIG_KEY_REPLAY_STATE`) so nested subgraphs load the
checkpoint that existed at the replay point on first visit, then resume
normal head loading within the same run.
- **Parent fork checkpoints on replay** — Time travel runs through
`PregelLoop._first()` (not `stream()` delegation on the parent
`Pregel`), creating an eager `source: "fork"` checkpoint and propagating
`ReplayState` to subgraphs.
- **Direct-to-subgraph time travel** — `getState()` subgraph delegation
is guarded with `CONFIG_KEY_READ`; direct subgraph configs strip stale
`RESUME` writes and prefer explicit `checkpoint_id` over
`checkpoint_map` when both are set.
- **Streaming** — Fixes subgraph interrupt namespace when streaming with
`subgraphs: true` (empty `checkpoint_ns` no longer becomes `[""]`;
parent emits interrupts under the deepest `checkpoint_map` namespace).

Closes #2325 (supersedes the earlier partial port).

### Implementation notes

| Area | Change |
|------|--------|
| `pregel/replay.ts` | New `ReplayState` class (mirrors Python) |
| `pregel/loop.ts` | Replay/time-travel detection, fork creation,
`RESUME` stripping, `ReplayState` wiring, stream namespace helpers |
| `pregel/index.ts` | `getState` subgraph delegation guard only (removed
`stream()` bypass that skipped parent fork creation) |
| Tests | `time_travel.test.ts` (14), `time_travel_extended.test.ts`
(33), shared `time_travel_helpers.ts`, Vitest matchers `toBeInterrupted`
/ `toHaveInterruptValue` |

---------

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