fix: clear setTimeout in buffered syncCollectionMethods with callbacks

[nexus-aissam]

Summary

When syncCollectionMethods (find, watch, aggregate) were called with a callback while buffering was enabled, the buffer timeout was never cleared after the operation completed successfully. This caused setTimeout references to accumulate in memory, leading to a memory leak.

The root cause was in lib/drivers/node-mongodb-native/collection.js lines 140-142, where the callback was not wrapped to call clearTimeout() - unlike all other buffered operation paths which properly clear the timeout.

Examples

Before this fix, the following code would leak memory:

const mongoose = require('mongoose');

// Connection not yet established - buffering enabled
const db = mongoose.createConnection();
const Model = db.model('Test', new mongoose.Schema({ name: String }));

// Each find() with callback creates a setTimeout that's never cleared
for (let i = 0; i < 1000; i++) {
  Model.find({}, (err, docs) => {
    // setTimeout reference leaks even after callback executes
  });
}

// Connect - operations execute but timeouts remain in memory
await db.openUri('mongodb://localhost/test');

After this fix, clearTimeout() is properly called when the operation completes, preventing the memory leak.

Fixes #15851