feat(checkpoint-mongo): add TTL support via MongoDB index

[sdip0971]

Description

This PR adds optional TTL (Time-To-Live) support to the MongoDB-backed checkpoint saver in the @langchain/langgraph-checkpoint package.

---

What's New

✅ Added enableTTL param in constructor:

export type MongoDBSaverParams = {
  client: MongoClient;
  dbName?: string;
  checkpointCollectionName?: string;
  checkpointWritesCollectionName?: string;

  // ✅ Change 1: Optional TTL toggle
  enableTTL?: boolean;
};

---

✅ Stored enableTTL in class and defaulted it:

this.enableTTL = enableTTL ?? false;

---

✅ Conditionally included TTL timestamp during checkpoint put:

const doc = {
  parent_checkpoint_id: config.configurable?.checkpoint_id,
  type: checkpointType,
  checkpoint: serializedCheckpoint,
  metadata: serializedMetadata,

  // ✅ Change 2: TTL timestamp only if enabled
  ...(this.enableTTL ? { _createdAtForTTL: new Date() } : {}),
};

---

Why This Matters

In long-running or high-frequency LangGraph applications, stale checkpoints can accumulate and consume storage.
This update allows users to opt-in to automatic cleanup by using MongoDB’s native TTL index on the _createdAtForTTL field.

---

Example TTL Index Creation (Manual Step)

// 🔁 This will auto-delete checkpoints after 1 hour
db.checkpoints.createIndex(
  { _createdAtForTTL: 1 },
  { expireAfterSeconds: 3600 }
);

---

Non-Breaking

• TTL is fully opt-in
• Existing functionality is untouched if enableTTL is not set
• Zero breaking changes for existing users or systems

---

Final Notes

Massive thanks to the LangGraph team for making such a cool platform 🙌
If you'd like to credit or tag me for the contribution, here’s my handle:

GitHub: @sdip0971

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[b-steel]

@sdip0971 Was just looking for this, thanks for beating me to the punch on it. Would it not be a good idea to include the checkpoint-writes collection in this as well so that a TTL index can be created there too?

Another note looking at your code, it seems like this includes your other PR, just FYI

[yishaistern]

@sdip0971 thanks for taking this 👍 , if it is ok to addd to what @b-steel wrote above , the DB for the checkpoint-writes is with a lot as docs as well