Remove long Iterator::chain sequence in RuleCodePrefix::iter (#10452

)

## Summary

This PR removes the `Iterator::chain(...)` sequence in
`RuleCodePrefix::iter()` with `Vec::expand` to avoid an
overlong-recursive types.

The existing `RuleCodePrefix::iter` method chains all rule group
iterators together. This leads to very long recursive types
`Chain<Map<Chain<Map<Chain<Map.....>>>>` (proportional to the number of
rule groups).

This PR rewrites the macro to use `Vec::extend` instead, which removes
the long recursive type (at the cost of introducing a potential
allocation).

## Alternatives

An alternative would be to use a stack allocated array by unrolling the
`Linter::iter` methods (generated by `EnumIter`).
I don't think it's worth the extra complexity, considering that
`RuleCodePrefix::iter` isn't a hot function.

## Test Plan

`cargo test`

crates/ruff_macros/src/map_codes.rs

@@ -391,8 +391,10 @@ fn generate_iter_impl(
             pub fn iter() -> impl Iterator<Item = RuleCodePrefix> {
                 use strum::IntoEnumIterator;
 
-                std::iter::empty()
-                    #(.chain(#linter_idents::iter().map(|x| Self::#linter_idents(x))))*
+                let mut prefixes = Vec::new();
+
+                #(prefixes.extend(#linter_idents::iter().map(|x| Self::#linter_idents(x)));)*
+                prefixes.into_iter()
             }
         }
     }