[Lex] `.$isTypeOf` is true when `$type` is undefined

[mozzius]

The generated .$isTypeOf utility returns true when $type is missing/undefined. This makes the check looser than implied. social-app's legacy type utility dangerouslyIsType requires $type to be present. We'd like this behaviour in Lex codegen output too

Relevant code

atproto/packages/lex/lex-schema/src/schema/typed-object.ts

Lines 86 to 90 in 5043c7a

	isTypeOf<TValue extends Record<string, unknown>>(
	value: TValue,
	): value is MaybeTypedObject<TType, TValue> {
	return value.$type === undefined || value.$type === this.$type
	}