Invoice is not generated when an UPCOMING_INVOICE dry run is triggered for an IN_ARREAR subscription with a TRIAL phase

[reshmabidikar]

Steps to reproduce:

1. Set date to 2023-01-01.
2. Upload the attached catalog.
3. Create a new account.
4. Create a subscription corresponding to the standard-monthly-in-arrear-with-trial plan.
5. Trigger invoice dry run of type UPCOMING_INVOICE as follows:

curl -v \
    -X POST \
    -u admin:password \
    -H "X-Killbill-ApiKey: bob" \
    -H "X-Killbill-ApiSecret: lazar" \
    -H "Content-Type: application/json" \
    -H "Accept: application/json" \
    -H "X-Killbill-CreatedBy: demo" \
    -H "X-Killbill-Reason: demo" \
    -H "X-Killbill-Comment: demo" \
    -d '{ "dryRunType": "UPCOMING_INVOICE"}' \
    "http://localhost:8080/1.0/kb/invoices/dryRun?accountId=xxx"  

6. No invoice is generated.
7. Move the clock to 2023-02-01.
8. Trigger invoice dry run again - An invoice is generated this time.

[reshmabidikar]

Update: After creating a test, the following was observed:

1. Although IN_ARREAR billing is used, the system generates an invoice corresponding to the TRIAL phase as soon as the subscription is created.
2. Thus, when the UPCOMING_INVOICE dry run is triggered, no invoice is generated (basically there is nothing to invoice for)
3. When the clock is moved by a month, the TRIAL phase ends. So triggering the UPCOMING_INVOICE dry run results in invoice generation.

So looks like the issue is at step 1, when IN_ARREAR billing is used, the invoice corresponding to the TRIAL phase should be generated after the TRIAL phase ends.