Code Completions Rest API - Add `X-GitLab-Error-Origin` header

What does this MR do and why?

Fixes #468347

This MR introduces a new header if the code_suggestions/completions REST api endpoint errors.

This is needed by EMs and PMs for reporting, see linked issue for the Why?.

MR acceptance checklist

✅

Screenshots or screen recordings

⛔

How to set up and validate locally

See example below, run the same or equivalent such that the API throws an error, then verify the error for the X-GitLab-Error-Origin header:

POST http://gdk.test:3000/api/v4/code_suggestions/completions
Accept: application/json
Authorization: Bearer glpat-...
Content-Type: application/json

{
  "current_file": {
    "file_name": "lol.js",
    "content_above_cursor": "function alertHelloWorld() {\n",
    "content_below_cursor": "\n}"
  },
  "stream": false
}