Fix an issue where JSON schema extras weren't JSON encoded

[dmontagu]

Closes fastapi/fastapi#10321

As far as I can tell, in v1, we would json encode anything extra (which included the examples). So I think this is restoring intended behavior (and as far as I can imagine is objectively better behavior in all circumstances where it might affect things).

@Kludex for reference

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[dmontagu]

Note this doesn't affect the result of a mutating json_schema_extra. To make that work would be a bit more complex because it makes in-place changes, so a straightforward call to to_jsonable_python won't work out of the box as that won't mutate the input. But it could be made to work, I'm just not sure it's worth the overhead it would add.