Option to not merge the base schema onto the input schema for `Model.discriminator`

[hasezoey]

Prerequisites

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🚀 Feature Proposal

A option to disable the current behavior of always merging the base schema onto the input schema

Motivation

Because when manually cloning the schemas beforehand this may result in duplicate things, or if not cloned at all from the base schema but with the same schema definition (not schema instance), it would create duplicate entries (for example hooks)

the current behavior runs into things like #12472

For example the documentation at Apply Plugins Before Compiling Models would not apply anymore with the current implementation

Example

With the option it would be up to the user to clone the schema or define a new schema from the ground up (with maybe a same schema definition), but still apply all required options for a discriminator

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fixes #12472