Add start_date and end_date args to subscriptionUsage

What does this MR do and why?

This adds start_date and end_date args to subscriptionUsage as part of https://gitlab.com/gitlab-org/gitlab/-/work_items/594756

Clients can now query subscription usage passing date range. Defaults to beginning and end of current month.

References

• Issue: https://gitlab.com/gitlab-org/gitlab/-/work_items/594756

Screenshots or screen recordings

N/A

How to set up and validate locally

1. Checkout this MR's steps to generate some data.
2. Open GraphQL and execute this query:

Query:

query getSubscriptionUsage($namespacePath: ID,  $startDate: ISO8601Date, $endDate: ISO8601Date) {
  subscriptionUsage(namespacePath: $namespacePath, startDate: $startDate, endDate: $endDate) {
    enabled
    isOutdatedClient
    lastEventTransactionAt
    startDate
    endDate
    purchaseCreditsPath
    creditsUsed
    overageTermsAccepted
    canAcceptOverageTerms
    dapPromoEnabled
    subscriptionPortalUsageDashboardUrl
    monthlyWaiver {
      totalCredits
      creditsUsed
      dailyUsage {
        date
        creditsUsed
      }
    }
    monthlyCommitment {
      totalCredits
      creditsUsed
      dailyUsage {
        date
        creditsUsed
      }
    }
    overage {
      isAllowed
      creditsUsed
      dailyUsage {
        date
        creditsUsed
      }
    }
    paidTierTrial {
      isActive
      dailyUsage {
        date
        creditsUsed
      }
    }
    usersUsage {
      totalUsersUsingCredits
      totalUsersUsingMonthlyCommitment
      totalUsersUsingOverage
      creditsUsed
      dailyUsage {
        date
        creditsUsed
      }
      users(first: 10, sort: TOTAL_CREDITS_USED_DESC) {
        pageInfo {
          hasNextPage
          hasPreviousPage
          startCursor
          endCursor
        }
        nodes {
          id
          name
          username
          avatarUrl
          usage {
            creditsUsed
            totalCredits
            monthlyCommitmentCreditsUsed
            monthlyWaiverCreditsUsed
            paidTierTrialCreditsUsed
            overageCreditsUsed
          }
          usedFlowTypes {
            id
            title
          }
          events(first: 5) {
            pageInfo {
              hasNextPage
              hasPreviousPage
              startCursor
              endCursor
            }
            nodes {
              timestamp
              eventType
              flowType
              creditsUsed
              sessionLink
              location {
                ... on Project {
                  id
                  name
                  fullPath
                }
                ... on Group {
                  id
                  name
                  fullPath
                }
              }
            }
          }
        }
      }
    }
  }
}

Variables:

{
  "namespacePath": "ultimate-group1",
  "startDate": "2026-03-01",
  "endDate": "2026-03-31"
}

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