[Code] Enforce progress update at the end of the index job

[mw-ding]

Summary

addressing https://github.com/elastic/code/issues/1561 and #42736 by adjusting progress reporting for index worker

Checklist

Use strikethroughs to remove checklist items you don't feel are applicable to this PR.

• This was checked for cross-browser compatibility, including a check against IE11
• Any text added follows EUI's writing guidelines, uses sentence case text and includes i18n support
• Documentation was added for features that require explanation or tutorials
• Unit or functional tests were updated or added to match the most common scenarios
• This was checked for keyboard-only and screenreader accessibility

For maintainers

• This was checked for breaking API changes and was labeled appropriately
• This includes a feature addition or change that requires a release note and was labeled appropriately

[elasticmachine]

Pinging @elastic/code

[elasticmachine]

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• continuous-integration/kibana-ci/pull-request