MIGRATIONS-2243 - Replace 'Shards' verbiage with 'Fragment' in console backfill status

[sumobrian]

Description

The customer-facing issue #1172 highlights confusion arising from the use of the term "shard" to describe both console backfill and console snapshot operations. The shard count in a snapshot does not necessarily match the shard count in a backfill. For instance, some indices (and their corresponding shards) may be excluded from the backill. To address this, we propose changing the terminology from "shards" to "worker items" for backfill operations. This change aligns with potential future updates where multiple worker items may be associated with a single shard.

• Category: Maintenance
• What is the old behavior before changes and new behavior after changes? console backfill status --deep-check displays items as "Work Items" instead of "Shards"

Issues Resolved

https://opensearch.atlassian.net/browse/MIGRATIONS-2243

Is this a backport? If so, please add backport PR # and/or commits #

Testing

[Please provide details of testing done: unit testing, integration testing and manual testing]

Check List

• New functionality includes testing
  • All tests pass, including unit test, integration test and doctest
• New functionality has been documented
• Commits are signed per the DCO using --signoff

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.
For more information on following Developer Certificate of Origin and signing off your commits, please check here.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 80.97%. Comparing base (3d82d1f) to head (43825de).
Report is 24 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff            @@
##               main    #1179   +/-   ##
=========================================
  Coverage     80.97%   80.97%           
  Complexity     2996     2996           
=========================================
  Files           407      407           
  Lines         15241    15241           
  Branches       1021     1021           
=========================================
  Hits          12342    12342           
  Misses         2272     2272           
  Partials        627      627           

Flag	Coverage Δ
unittests	80.97% <ø> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

[AndreKurait]

LGTM