feat(ai): added 2020-12 schema migration to ai strict validation
- Added draft-07-to-2020-12 schema upgrade logic for `$schema`, `definitions`, `$ref`, tuple, and nullable mapping.
- Integrated `upgradeJsonSchemaTo202012` into strict adaptation, validation, strict-mode, sanitizer, and wire/dereference flows.
- Replaced set-based traversal helpers with record maps, `for...in` loops, and epoch/`once` cycle guards.
- Added unreleased changelog entries and tests for schema normalization, `oneOf` to `anyOf`, and tuple validation edge cases.