Fix RBS Record translation to include optional fields

[Morriar]

Summary

• RBS Record types with optional fields (e.g. { a: Integer, ?b: String }) were silently dropped during translation to RBI
• The translator used type.fields which only returns required fields — switched to type.all_fields which includes both required and optional fields
• Optional fields are now wrapped in T.nilable to represent that they may not be present
• Already-nilable optional fields (e.g. ?c: Float?) are not double-wrapped thanks to Type.nilable simplification

[paracycle]

Suggested change

	Type.shape(type.all_fields.map do |name, (value, required)|
	translated = translate(value)
	[name, required ? translated : Type.nilable(translated)]
	end.to_h)
	Type.shape(type.all_fields.to_h do |name, (value, required)|
	translated = translate(value)
	[name, required ? translated : Type.nilable(translated)]
	end)