fix: handle removal of :context multitenancy in migrations

[Ryan6794]

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• Bug fixes include regression tests
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When a resource transitions from :context multitenancy to global?: true (or removing multitenancy entirely), the migration generator produces an invalid migration that tries to drop a foreign key constraint that doesn't exist yet.

Bugs:

1. Wrong snapshot folder lookup - :context snapshots are stored in tenants/ but after removing multitenancy the code looks in the base folder. The old snapshot is not found so the table is treated as brand new.

2. Incorrect FK drop generated - has_reference?/2 did not account for the case where both the table and the referenced resource previously had :context multitenancy. Since the FK was never created in the global schema, it should not be dropped.

Fix:

1. get_existing_snapshot/2: fall back to checking the alternate folder when the primary lookup returns nil.
2. has_reference?/2: return false when both multitenancy strategies are :context, since no global FK exists to drop.

Testing:

Reproduced using the demo repo linked in the issue: https://github.com/serpent213/ash_postgres_multitenancy_migration_demo

After the fix, all three steps complete successfully:

1. mix ash.reset
2. mix ash_postgres.generate_migrations --dev
3. mix ash.migrate

[zachdaniel]

Could we add a test for this too?