feat: send receipt email on non-zero invoice total

[iloveagent57]

We only want to send a receipt email when the invoice.paid total is > 0.

Merge checklist:

• ./manage.py makemigrations has been run
  • Note: This must be run if you modified any models.
    • It may or may not make a migration depending on exactly what you modified, but it should still be run.

Post merge:

• Ensure that your changes went out to the stage instance
• Deploy to prod instance

[Copilot]

Pull request overview

This PR conditionally sends receipt emails only when the invoice.paid total is greater than zero, preventing emails from being sent during free trial periods.

Key changes:

• Added conditional logic to check invoice.total > 0 before sending payment receipt emails
• Enhanced test coverage to verify email sending behavior for both zero and non-zero invoice totals
• Updated test helper to default invoice total to 0 for invoice.paid events

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 2 comments.

File	Description
enterprise_access/apps/customer_billing/stripe_event_handlers.py	Added conditional check to only send receipt emails when invoice total > 0
enterprise_access/apps/customer_billing/tests/test_stripe_event_handlers.py	Added test case for non-zero invoice total, updated test helper to default total to 0, and added assertions to verify email sending behavior

---

💡 Add Copilot custom instructions for smarter, more guided reviews. Learn how to get started.