Add invoice.upcoming trigger support via test clock fixture

[SumitKumar-17]

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cc @stripe/developer-products

Summary

Fixes the issue where stripe trigger invoice.upcoming failed
with "event is not supported", even though stripe listen --events invoice.upcoming
already worked.

Root cause: invoice.upcoming is a system-generated event fired by
Stripe's billing engine 3 days before a subscription renews.

Fix: Add pkg/fixtures/triggers/invoice.upcoming.json using Stripe's
test clock API — the approach Stripe's own documentation recommends for
testing time-based billing events.

The fixture:

1. Creates a test clock frozen at Jan 1, 2024
2. Creates a customer tied to that test clock (with pm_card_visa as default payment method)
3. Creates a product + monthly recurring price
4. Creates a subscription (first invoice paid immediately at Jan 1)
5. Advances the clock to Jan 29, 2024 — 3 days before the Feb 1 renewal —
   which causes Stripe's billing engine to emit invoice.upcoming

Using static timestamps keeps the fixture deterministic.

Verified end-to-end against live test keys:

Stripe returned evt_... with "type": "invoice.upcoming",
"billing_reason": "upcoming", and a draft invoice (no ID) attached —
exactly the payload a real invoice.upcoming webhook carries.

Changes

• pkg/fixtures/triggers/invoice.upcoming.json — new fixture file

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[SumitKumar-17]

If Approved adds the invoice.upcoming triggers also fixes #558