Follow-up from "feat: add /fix with duo quick chat "source" code action"

The following discussions from !2585 (merged) should be addressed:

• @elwyn-gitlab started a discussion:

  Note: updated expected values since we use the more complete mocks now which correctly account for 0 based vs 1 based lines.

• @erran started a discussion: (+1 comment)

  question: .filter(Boolean) but casting to code actions after? Should we filter out falsy items instead?

• @erran started a discussion:

  How many lines does an empty file have?

• @erran started a discussion:

  non-blocking: could we use a jest matched to assert we have an object with these attributes in case we introduce another action which is prepended to the list in the future?