Switch the default uuidRepresentation from "pythonLegacy" to "unspecified" (not "standard")

First off, I want to apologize for this whole uuidRepresentation situation. The issue stems from a mistake made in certain MongoDB drivers a decade ago and it could have been addressed a long time ago. I'm sorry that this problem has bubbled over into mongoengine.

I noticed this uuidRepresentation warning which states that mongoengine plans to switch the default uuidRepresentation from "pythonLegacy" to "standard":

    if "uuidrepresentation" not in keys:
        warnings.warn(
            "No uuidRepresentation is specified! Falling back to "
            "'pythonLegacy' which is the default for pymongo 3.x. "
            "For compatibility with other MongoDB drivers this should be "
            "specified as 'standard' or '{java,csharp}Legacy' to work with "
            "older drivers in those languages. This will be changed to "
            "'standard' in a future release.",
            DeprecationWarning,
        )
        kwargs["uuidRepresentation"] = "pythonLegacy"

I'm concerned about this plan and the pain it will cause users. In particular, switching the default from "pythonLegacy" to "standard" will mean that application UUID data will be silently rewritten from subtype 3 to 4 (AKA a mild form of data corruption) and queries may break.

I believe the only safe decision is for mongoengine to either keep using "pythonLegacy" as the default or switch to "unspecified". Switching to "unspecified" avoids data corruption by forcing the user to make a choice if they want to encode UUIDs. They'll need to explicitly add "pythonLegacy" if they want to work with UUIDs already stored with the default behavior, or switch to "standard" if they are storing UUIDs for the first time. Applications also have the ability to migrate from "pythonLegacy" to "standard" if they rewrite all UUID data (or adjust their queries to match either subtype 3 or 4). This is why we switched the default in PyMongo from "pythonLegacy" to "unspecified".

For more background see: https://pymongo.readthedocs.io/en/stable/examples/uuid.html

Please let me know if you have any questions.

Start by reviewing the uuidRepresentation warning shown in the issue and the linked PyMongo UUID documentation. Trace where MongoEngine sets this default and identify the related tests or documentation before changing behavior. Done means the default is unspecified, compatibility concerns are covered, and the expected UUID behavior is verified.