feat: consistently define fetch_multi_by

[cribbles]

Motivation

Following Shopify/identity_cache#534, IdentityCache will define fetch_multi_by for cache_index invocations regardless of the number of keys included in the index. (Previously only single-index keys were supported.)

We should lift the field_length restriction accordingly.

Implementation

Removes the field_length == 1 restriction (as well as the field_length variable designation, which is now useless).

Tests

Nothing changed.

Updated a unit test implicitly testing this behavior in 64983d4.

[dylanahsmith]

My inclination would have been to remove any tests related to the field length restriction, tbh, but there weren't any, and I see no reason to extend tests beyond the happy path in this case. I am open to changing this if a suitable approach is proposed in the reviews.

Looks like there was a test for the absence of this generated method that can be changed to expect its presence. The test failure is

          it generates methods for combined cache_indexes                 FAIL (0.84s)
        --- expected
        +++ actual
        @@ -20,6 +20,9 @@
             sig { params(index_values: T::Enumerable[T.untyped], includes: T.untyped).returns(T::Array[::Post]) }
             def fetch_multi_by_title(index_values, includes: nil); end
         
        +    sig { params(index_values: T::Enumerable[T.untyped], includes: T.untyped).returns(T::Array[::Post]) }
        +    def fetch_multi_by_title_and_review_date(index_values, includes: nil); end
        +
             sig { params(keys: T::Enumerable[T.untyped]).returns(T::Array[::Integer]) }
             def fetch_multi_id_by_title(keys); end
           end
        /home/runner/work/tapioca/tapioca/spec/tapioca/dsl/compilers/identity_cache_spec.rb:223:in `block (3 levels) in <class:IdentityCacheSpec>'

[cribbles]

@dylanahsmith d'oh. Corrected in 64983d4

[paracycle]

Thank you!