get_join_arel_node fails with include_optional_linkage_data if there is already a join

[pjungwir]

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Description

If the user adds a join (e.g. in records) and that association has include_optional_linkage_data: true, then get_join_arel_node prints "get_join_arel_node: No join added" to stdout, returns nil, and a little later we crash because of the nil.

This happens because get_join_arel_node compares the node count before & after adding a join, but the count doesn't change because the join was already there.

We should fix this by looking for a compatible join and only failing if we can't find one. If we are trying to add a left outer join, then any join type will do (since adding an outer join wouldn't change the cardinality). If we are trying to add an inner join, then only another inner join is compatible (since otherwise the filtering we expect wouldn't happen).

I have a test case in my related PR, so I haven't included one here.

I hit this bug on master (a3a2a7a) and ruby 3.1.2p20.