IdentityCache compiler generates wrong return types for fetch_multi_n_by_m fields

[bitwise-aiden]

Description

After implementing the missing fetch_multi_n_by_m methods in #1758 I have found out that the method signature isn't always correct as shown in this example from cache_attribute: https://github.com/Shopify/identity_cache/blob/main/lib/identity_cache/cached/attribute.rb#L82-L101

The result of methods that include by in the name is a hash where:

• Key: type m (be it a single value or an array of multiple)
• Value: type n or T::Array[n] depending on things like if unique has been passed to cache_attribute.

So at present when the return type is T::Array[n], it would be expected that it is actually T::Hash[T.untyped, n] or T::Hash[T.untyped, T::Array[n]] based on how it was created.

[andyw8]

@KaanOzkan @Morriar So as not hold it up any further, I've marked two calls in the latest Core RBI update as T.unsafe, with smart TODOs pointing at this issue.