MongoDB regression: ISODate fields get persisted as strings

This issue was introduced in v4.26.0 (commit 9f692589b). Steps to reproduce:

> docker run --rm -p27017:27017 --name mongodb -e MONGO_INITDB_ROOT_USERNAME=mongoadmin -e MONGO_INITDB_ROOT_PASSWORD=secret mongo
> docker exec -it mongodb bash
>> mongosh -u mongoadmin -p secret
>>> use food
>>> db.createCollection("fruits")

input:
  generate:
    mapping: |
      root.ts = "2024-04-29T12:34:05".ts_strptime("%Y-%m-%dT%T")
      root.name = "apple"
    count: 1

output:
  mongodb:
    url: mongodb://localhost:27017/?maxPoolSize=3
    username: mongoadmin
    password: secret
    database: food
    collection: fruits
    operation: insert-one
    write_concern:
      w: "majority"
      j: false
      w_timeout: 5s
    upsert: false
    document_map: |
      root = this

Run the above config with Benthos v4.25.0 and then with the current main branch (commit 51d971a) and then execute db.fruits.find() in mongosh. You'll notice that the ts field is different for the two entries.

Reproduce the regression with the provided Docker, MongoDB, and Benthos configurations, comparing v4.25.0 with commit 51d971a. Start by tracing the MongoDB output path and document_map handling for the ISODate value introduced in v4.26.0. Done means the timestamp is persisted consistently as a MongoDB date rather than a string, with the regression covered by a test.