Sum all credit grants of an Orbit trial

What does this MR do and why?

When Sales adds credits to an Orbit trial, CustomersDot records each grant as its own trial entry. GitLab read only the first entry, so the Orbit settings page showed the first grant's credits, and the trial dates and state followed that grant. A new trial that Sales starts after an earlier trial ended also showed as ended.

• The current trial is the set of grants with the latest expiry. CustomersDot gives all active grants of a trial the same expiry, so grants that expire earlier belong to an earlier trial.
• The credit total is the sum of the current trial's grants, and the start date is its earliest grant.
• The end date and the active or ended state come from the current trial. A self-serve trial has one grant, so nothing changes for it.

References

Closes https://gitlab.com/gitlab-org/gitlab/-/work_items/635709

Screenshots or screen recordings

E2E run against the local CustomersDot, with the grants written to the group's Orbit trial wallet. Before: master. After: this branch.

	Before	After
Grants of 50, 10, 10 and 10 credits (the issue's wallet)
An earlier trial of 50 credits expired, then Sales started a new trial with 10 credits

Wallet	Before	After
50 + 10 + 10 + 10, one trial	trial with 50 credits	trial with 80 credits
Earlier trial of 50 expired, new trial of 10	trial ended, Orbit paused	trial with 10 credits, active
50 + 10, all expired	trial ended	trial ended
One grant of 100	trial with 100 credits	trial with 100 credits

The hourly ended-trials cron now keeps Orbit on for the new trial, and still turns it off when all grants have expired.

Regression: all 29 scenarios of the Orbit monetization E2E factory pass on this branch, the same as on master. After the current-trial change, the trial scenarios (start, end, and sales-assisted, on GitLab.com and Self-Managed) pass again.

How to set up and validate locally

1. In a Rails console, parse a CustomersDot payload with an expired trial and a new trial:

   payload = { 'trial_info' => { 'orbit' => { 'current_balance' => '10.0', 'trials' => [
     { 'starts_at' => 2.months.ago.iso8601, 'expires_at' => 1.month.ago.iso8601, 'initial_credit_amount' => '50.0' },
     { 'starts_at' => 1.day.ago.iso8601, 'expires_at' => 1.month.from_now.iso8601, 'initial_credit_amount' => '10.0' }
   ] } } }
   trial = Gitlab::SubscriptionPortal::Client.send(:parse_product_trial, :orbit, payload)
   [trial.state, trial.credits_total, trial.trial_expires_at] # => [:trial, 10.0, <one month from now>]

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