Search API v2: Add adapter for autocomplete

[csutter]

This brings #1282 back around now that we have decided not to expose Search API v2's autocomplete endpoint publicly, and need to access it from Finder Frontend.

This adds a method for the new autocomplete functionality to Search API v2's API adapter.

• Add SearchApiV2#autocomplete
• Remove redundant error scenario tests to avoid tests bloating up too much

[kevindew]

Looks good, though I imagine that this might box in for local dev where the public endpoints aren't accessible. Based on our previous chat about /api/search/autocomplete.json and /api/search.json, we'd find that a local dev could only have one of these. Is this a problem?

Also, only loosely related to this PR but a reminder before I forget. I guess autocomplete could push a lot more traffic through finder-frontend so we may want to up the number of pods.

[csutter]

Looks good, though I imagine that this might box in for local dev where the public endpoints aren't accessible. Based on our previous chat about /api/search/autocomplete.json and /api/search.json, we'd find that a local dev could only have one of these. Is this a problem?

So this should work in local dev if the developer has search-api-v2 running, which is now less of a pain than it used to be – and if they don't, I reckon autocomplete is far enough off the critical path for local development that it'll be fine (worst outcome is you'll get some fetch errors in the console).

Also, only loosely related to this PR but a reminder before I forget. I guess autocomplete could push a lot more traffic through finder-frontend so we may want to up the number of pods.

Agree, they're relatively basic requests but might block enough of the capacity that it's worth dialling up in anticipation 📈