cope with both variable length uri_path (might or might not have /archive/ in) and that size may be NNNpx or prefix-NNNpx

ArchivedPublic

Actions

(spark 3.3.0 gains the split_part function, which would make this rather simpler)

We want the final element of uri_path split by / (it's not a fixed length because of archive thumbs).
Then to take the string up to "px-" (which is usually just the size) hence splitting on "px-" and taking the first element.
The complication is that there are a number of prefixes that might come before the size (e.g. langfr-250px for a translated SVG file), so we then want the last element of that string split on '-'.

Now obsoleted by a regexp-based approach (see https://phabricator.wikimedia.org/T410304#11383363)