Follow-up from "Use Duo Chat for explain code feature"
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### Summary
This is a follow-up from !143217 to pass `contentAboveCursor` and `contentBelowCursor` in the GraphQL mutation when using the Explain Code feature. This should put is closer to how explain code works in VSCode.
The following discussion from !143217 should be addressed:
- [ ] @dmishunov started a [discussion](https://gitlab.com/gitlab-org/gitlab/-/merge_requests/143217#note_1751744180):
> **note:** The `chat` action also accepts `contentAboveCursor` and `contentBelowCursor` as part of the `currentFile`prop as defined in https://gitlab.com/gitlab-org/gitlab/-/blob/master/ee/app/graphql/types/ai/current_file_input_type.rb. Do we plan on including those as well to have the feature 100% identical to `/explain` in VSCode?
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