fix: remove admin summary from emails sent counter

[davidgamez]

Summary:

This pull request improves the accuracy of notification delivery statistics by ensuring that admin summary emails (admin.event_summary) are excluded from delivery counts, as including them could mislead administrators. It also introduces a dedicated test to verify this behavior.

Notification delivery statistics improvements:

• Updated the _aggregate_delivery_stats function in dispatch_monitor.py to exclude admin.event_summary notifications from delivery statistics by joining with NotificationEvent and filtering out those with the ADMIN_EVENT_SUMMARY type. [1] [2]

Testing enhancements:

• Added the TestAggregateDeliveryStats test case in test_dispatch_notifications.py to ensure that admin summary deliveries are not counted in delivery statistics, including setup and teardown logic for test isolation.
• Imported the updated _aggregate_delivery_stats function into the test module for direct testing.

Expected behavior:

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Testing tips:

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Please make sure these boxes are checked before submitting your pull request - thanks!

• Run the unit tests with ./scripts/api-tests.sh to make sure you didn't break anything
• Add or update any needed documentation to the repo
• Format the title like "feat: [new feature short description]". Title must follow the Conventional Commit Specification(https://www.conventionalcommits.org/en/v1.0.0/).
• Linked all relevant issues
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[cka-y]

LGTM