`json`: fix additionalProperties, allow space after enum/const (#7840)

* json: default additionalProperty to true

* json: don't force additional props after normal properties!

* json: allow space after enum/const

* json: update pydantic example to set additionalProperties: false

* json: prevent additional props to redefine a typed prop

* port not_strings to python, add trailing space

* fix not_strings & port to js+py

* Update json-schema-to-grammar.cpp

* fix _not_strings for substring overlaps

* json: fix additionalProperties default, uncomment tests

* json: add integ. test case for additionalProperties

* json: nit: simplify condition

* reformat grammar integ tests w/ R"""()""" strings where there's escapes

* update # tokens in server test: consts can now have trailing space