feat: validate enum values for properties and collections

[crisnahine]

rebased and fixed version of #184 by @dark-panda.

adds enum validation for property and collection DSL on tools. properties use ActiveModel inclusion validation, collections use a custom ArrayEnumValidator that checks each item.

fix from the original: collection_definition.merge!(opts) was putting enum at the array level in the JSON Schema output. in JSON Schema, enum on an array means the entire value must be one of the listed values — not that each item should be. moved it to items where it belongs:

# before (wrong — enum on array level)
{ type: "array", items: { type: "string" }, enum: ["apple", "banana", "cherry"] }

# after (correct — enum on items)
{ type: "array", items: { type: "string", enum: ["apple", "banana", "cherry"] } }

also added nil guard in ArrayEnumValidator and cleaned up a stale comment.