ActiveRecord find_nth finder respects limit()

[brchristian]

Summary

Fixes #23979.

Issue #23979 describes behavior where the ActiveRecord first and last would behave differently following a limit(), and DHH's comment in #23598 (comment) pointed to the first method being the one that was behaving incorrectly.

Specifically, limit(1).first(2) would return a set of size 2 (incorrect), while limit(1).last(2) would return a record set of size 1 (correct).

This PR introduces a test suite to capture the interrelationships between limit and the finder methods (both first(n) as well as second, third, and so on).

It also adjusts those finder methods to make the test suite pass: making them respect the limit and making them consistent with the behavior of the last finders.

[rails-bot]

r? @schneems

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[brchristian]

@schneems Let me know if there’s anything else I can do to help prepare this PR!

[schneems]

@sgrif missed this one coming in awhile ago, can you take a look?

[brchristian]

The change in c9220bf required a significant refactor, so I am closing this PR in favor of #27597.