fix: correlate bulk upsert records by identity on the MERGE path

[weljoda]

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Summary

Ash.bulk_create/4 with upsert? true plus a manage_relationship/3 change that creates a has_one child can attach children to the wrong parents on PostgreSQL 17+.

The correct logic was already in place, but dead code, because it didn't use the correct variable. So the fix is a simple one line, using the correct variable options instead of opts. The rest is a regression test to reproduce the error.

Because the error is specific to Postgres 17/18 I couldn't reproduce it by running mix test also running it with --include postgres_17 didn't work. I needed to run PG_VERSION=17 mix test --include postgres_17 to reproduce it.