Function inferJsonSchemaDraft

Infer the JSON Schema draft from keyword presence when $schema is absent. Examined from highest-confidence to lowest.

Heuristics:

1. $dynamicRef / $dynamicAnchor / prefixItems → Draft 2020-12
2. $recursiveRef / $recursiveAnchor / unevaluatedProperties / dependentSchemas → Draft 2019-09
3. if / then / else, contentEncoding / contentMediaType → Draft 07
4. const, examples (array), propertyNames → Draft 06
5. Boolean exclusiveMinimum, id (no $), definitions only → Draft 04
6. No signal → Draft 2020-12