feat(checkpoint-mongodb): add TTL support for automatic checkpoint data expiration

[Mihailoff]

Add optional TTL parameter to MongoDBSaver that enables automatic expiration of checkpoints using MongoDB's TTL indexes.

• Add ttl option to MongoDBSaverParams (value in seconds)
• Add setup() method to create TTL indexes, returns Error[] for caller to handle failures
• Add upserted_at timestamp to documents when TTL is enabled
• Each write refreshes TTL (expires after inactivity, not creation)

  // Create checkpointer with 1-hour TTL (in seconds)                         
  const checkpointer = new MongoDBSaver({                                     
    client,                                                                   
    ttl: 3600,                                                                
  });                                                                         
                                                                              
  // Create TTL indexes (call on startup/deployment)                          
  const errors = await checkpointer.setup();                                  
  if (errors.length) {                                                        
    console.warn("Failed to create TTL indexes:", errors);                    
  } 

Fixes # (issue)

Ever growing records in database.

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🦋 Changeset detected

Latest commit: 59e82a4

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 1 package

Name	Type
@langchain/langgraph-checkpoint-mongodb	Minor

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[christian-bromann]

LGTM 👍

@hntrl @dqbd mind taking a 2nd look?