Audit and validate the MongoDB backend

[jamessewell]

If you're a MongoDB expert, we'd love your help reviewing our MongoDB driver.

What this driver does

The MongoDB driver (backends/mongodb/) is a standalone backend using the official go.mongodb.org/mongo-driver.

It implements the Driver interface:

• Exec — parses JSON config to drop collections, create Atlas Search indexes, and waits (polling every 2s, up to 120s) for search indexes to reach READY status
• Query — runs $search aggregation pipelines and counts result documents
• Insert — bulk inserts using InsertMany, mapping id columns to _id
• Update — upserts using BulkWrite with ReplaceOne models and upsert=true
• CaptureConfig — runs buildInfo admin command to capture version, storage engines, allocator, and JS engine info

Pool: MaxPoolSize=20, MinPoolSize=5. Adds directConnection=true for local URIs. 10s connect timeout.