test: reproduce parent() scoping bug in nested exists with calculation

[barnabasJ]

Bug Description

When a calculation on resource B uses parent() inside an unrelated exists(), and resource A references that calculation via exists(relationship_to_b, b_calculation), the parent() references incorrectly resolve to resource A instead of resource B.

Reproduction

Post has a calculation using an unrelated exists with parent():

calculate :has_matching_author_by_unrelated_exists,
          :boolean,
          expr(exists(Author, first_name == parent(title)))

When loaded directly on a Post, parent(title) correctly refers to Post.title.

Author has a calculation that uses the Post calculation via a relationship exists:

calculate :has_post_matching_author_via_nested_exists,
          :boolean,
          expr(exists(posts, has_matching_author_by_unrelated_exists))

When loaded on Author, the generated SQL resolves parent(title) to authors.title instead of posts.title:

-- Generated (incorrect): authors.title
SELECT ... exists(... ssa0."first_name"::text = a0."title"::text ...)
--                                               ^^^ a0 = authors table

-- Expected: posts.title
SELECT ... exists(... ssa0."first_name"::text = sp0."title"::text ...)
--                                               ^^^ sp0 = posts table

What's Included

• Failing test demonstrating the bug
• Passing sanity test confirming the calculation works correctly when loaded directly on the child resource
• Test resource additions (calculation on Post and Author)

Impact

This prevents using exists(relationship, calculation) when the calculation itself contains parent() references inside an unrelated exists(). The workaround is to inline the calculation logic with explicit parent scoping or use a module calculation instead.