BBM - Estimate time remaining follow up

Ref: #589188 (closed)

What does this MR do and why?

Following up !221255 (merged)

This MR immproves estimated time remaining display and error handling.

• CLI improvements: Combines progress and estimated time remaining into a single column for better readability
• Admin UI: Adds estimated time remaining to the background migrations view
• Error handling: Adds rescue mechanism to handle query failures gracefully

Testing

• Create a new BBM
• Run CLI command
• Visit admin UI

Click to expand

Gitlab::Database::SharedModel.using_connection(ActiveRecord::Base.connection) do
  migration = Gitlab::Database::BackgroundMigration::BatchedMigration.create!(
    job_class_name: 'TestMigration',
    table_name: 'users',
    column_name: 'id',
    interval: 120,
    min_value: 1,
    max_value: 1000,
    batch_size: 100,
    sub_batch_size: 10,
    total_tuple_count: 100,
    status: 1,
    gitlab_schema: :gitlab_main
  )
  
  migration.batched_jobs.create!(
    status: 3,
    batch_size: 50,
    sub_batch_size: 10,
    pause_ms: 100,
    min_value: 1,
    max_value: 50,
    started_at: 60.seconds.ago,
    finished_at: Time.current
  )
end

➜  gitlab git:(morefice/bbm-estimate-time-remaining-2) ✗ bundle exec rake gitlab:background_migrations:list
id     | table_name                    | job_class_name                                       | status   | progress
-------|-------------------------------|------------------------------------------------------|----------|---------------------------------------------
main_1 | plan_limits                   | BackfillPlanLimitsPlanNameUid                        | finished | 100.00%
main_2 | gitlab_subscriptions          | BackfillGitlabSubscriptionsHostedPlanNameUid         | finished | 100.00%
main_3 | gitlab_subscription_histories | BackfillGitlabSubscriptionHistoriesHostedPlanNameUid | finished | 100.00%
main_4 | users                         | TestMigration                                        | active   | 50.00% (estimated time remaining: 2 minutes)
ci_1   | ci_pending_builds             | BackfillCiPendingBuildsPlanNameUid                   | finished | 100.00%
ci_2   | ci_runners                    | BackfillCiRunnersAllowedPlanNameUids                 | finished | 100.00%