Fix JsonSchema strict to be encoded even if it's default value is true

[LittleTrickster]

Q	A
Bug fix?	yes
New feature?	no
BC breaks?	no

Describe your change

1. Make strict to be encoded when it is set to true in JsonSchema
2. Add additionalProperties to jsonSchema() test, as it is required when strict is true

What problem is this fixing?

strict true in JsonSchema was never encoded to json since kotlin Json does not encode default values so openai thinks its false by default
When strict is false and schema has nested classes or lists then ChatMessage content gets wrapped in extra object like content": "{\"type\":\"object\",\"properties\":{\"isValid\":true,...
instead of expected "content": "{\"isValid\":true,...