Use proper order of `Completion::Source` field to have sane default (#26416)

Follow-up of https://github.com/zed-industries/zed/pull/26300

Release Notes:

- N/A
d81e7683eaKirill Bulatov committed on 3/10/2025, 10:58:21 PM· committed by GitHubparent8b29ee6
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