Add securityPoliciesSyncStatus GraphQL query

What does this MR do and why?

We are working on visualising the synchronisation status of merge request approval policies in the policy overview UI (&19049).

Currently we only publish updates during the sync through the GraphQL subscription introduced by !201777 (merged). Since we have the product requirement that sync status remains accessible for 24 hours, this merge request:

• only partially clears Redis state once policy sync completes
• adds a securityPoliciesSyncUpdated GraphQL query that returns the same data structure as the subscription
• both the query and the subscription now contain an inProgress field that signals whether a sync is ongoing

In !202376, the progress bar UI will use this query to hydrate itself on page load.

References

• #562958 (closed)

How to set up and validate locally

Follow How to set up and validate locally but use the following subscription and query:

query {
  securityPoliciesSyncStatus(policyConfigurationId: "gid://gitlab/Security::OrchestrationPolicyConfiguration/$CONFIGURATION_ID") {
    projects
    projectsTotal
    failedProjects
    mergeRequests
    mergeRequestsTotal
    inProgress
  }
}

subscription {
  securityPoliciesSyncUpdated(policyConfigurationId: "gid://gitlab/Security::OrchestrationPolicyConfiguration/$CONFIGURATION_ID") {
    projectsProgress
    projectsTotal
    failedProjects
    mergeRequestsProgress
    mergeRequestsTotal
    inProgress
  }
}

MR acceptance checklist

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Related to #562958 (closed)