Refactor `codegen_deferred_annotations_body` to ensure `mangled` is decref'd

[A0su]

Bug report

Bug description:

Follow-up item from this PR: #146443 (review)

During error cases we may fail to decref mangled while using these ADDOP* and VISIT macros. As a result, we can extract the for loop's body into a subfunction that we check the return of and decref mangled if needed.

        // NOTE: ref of mangled can be leaked on ADDOP* and VISIT macros due to early returns
        // fixing would require an overhaul of these macros

CPython versions tested on:

CPython main branch

Operating systems tested on:

Windows

Linked PRs

• gh-148059: Refactor codegen_deferred_annotations_body to avoid missing decref of mangled #148060