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A base pydantic.BaseModel for all Prefect schemas and pydantic models.

As the basis for most Prefect schemas, this base model usually ignores extra fields that are passed to it at instantiation. Because adding new fields to API payloads is not considered a breaking change, this ensures that any Prefect client loading data from a server running a possibly-newer version of Prefect will be able to process those new fields gracefully. However, when PREFECT_TEST_MODE is on, extra fields are forbidden in order to catch subtle unintentional testing errors.