Fix Ash.Reactor notification_metadata for bulk_create and bulk_update

[meeq]

• Added tests for notification_metadata on bulk_create and bulk_update Ash.Reactor steps
• Fixed support for Reactor template values with notification_metadata on bulk_create and bulk_update Ash.Reactor steps
• Unified logic for Ash.Notifier.Notification instantiation

Contributor checklist

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• I accept the AI Policy, or AI was not used in the creation of this PR.
• Bug fixes include regression tests
• Chores
• Documentation changes
• Features include unit/acceptance tests
• Refactoring
• Update dependencies

[zachdaniel]

Please run mix format and mix spark.cheat_sheets (that second one may be an issue from main?).

@jimsynz would you mind taking a look?