[Fix #40109] Incorrect joins for polymorphic has_many through: with association scope

[joshuay03]

Motivation / Background

Fixes #40109

Detail

Updates ActiveRecord::Reflection::PolymorphicReflection#join_scopes to not apply the previous reflection's join scopes if it's a through reflection, as the scope will eventually be built in the context of the polymorphic reflection.

Additional information

Checklist

Before submitting the PR make sure the following are checked:

• This Pull Request is related to one change. Unrelated changes should be opened in separate PRs.
• Commit message has a detailed description of what changed and why. If this PR fixes a related issue include it in the commit message. Ex: [Fix #issue-number]
• Tests are added or updated if you fix a bug or add a feature.
• CHANGELOG files are updated for the changed libraries if there is a behavior change or additional feature. Minor bug fixes and documentation changes should not be included.

[joshuay03]

Before patch:

[joshuay03]

SQL comparison

Before:

SELECT posts.*
FROM posts
INNER JOIN categories_posts
    ON categories_posts.post_id = posts.id
INNER JOIN categories
    ON categories.id = categories_posts.category_id
INNER JOIN essays
    ON essays.name = 'Bob'
    AND essays.writer_type = 'Author'
    AND essays.category_id = categories.name
INNER JOIN authors
    ON authors.name = 'Bob'
    AND authors.name = essays.writer_id

After:

SELECT posts.*
FROM posts
INNER JOIN categories_posts
    ON categories_posts.post_id = posts.id
INNER JOIN categories
    ON categories.id = categories_posts.category_id
INNER JOIN essays
    ON essays.writer_type = 'Author'
    AND essays.category_id = categories.name
INNER JOIN authors
    ON authors.name = 'Bob'
    AND authors.name = essays.writer_id