Refactor count workorders to reuse search code

[jyeshe]

Validation Steps

$ mix test test/lightning/digest_email_worker_test.exs

Notes for the reviewer

As a continuation of #2118:

• Reuse search_workorders_query for counting
• Reuse search_workorders_query to return workorders for retries (with dataclips not wiped)

Keeps same behavior as https://github.com/OpenFn/lightning/pull/2123.

Related issue

Refs #2121

Review checklist

• [x] I have performed a self-review of my code
• [x] I have verified that all appropriate authorization policies (:owner, :admin, :editor, :viewer) have been implemented and tested
• [x] If needed, I have updated the changelog
• [ ] Product has QA'd this feature