Cover API example sideways scrolling with a browser journey #4847

A browser journey should fail if long lines in the API examples rail stop being scrollable with a mouse, or if the line numbers or the "Generated from the API spec" note stop staying in place.

What is getting in your way

#4843 fixes these with CSS only: a draggable scrollbar on the code cards, a sticky line-number gutter, and a sticky note in responses. The carousel tests run in jsdom with stubbed sizes, and the only API journey (api-explorer/synth/browse-api) never scrolls an example. Nothing checks the real layout, so a later CSS change can bring the invisible scrollbar or the drifting gutter back while mise run site:test still passes. The review on that pull request asked for this coverage: #4843 (comment)

What would you like instead

A journey that opens an API operation with an overflowing request sample and an overflowing generated response, then for each one:

• drags the code card's scrollbar sideways,
• checks that the code scrolled and the carousel kept the same language,
• checks that the line-number gutter and the "Generated from the API spec" note are still at the left and bottom-right of the visible code.

Anything else

Two open questions decide the shape:

• The journeys use click, expect, get, wait and snapshot today. A drag on a scrollbar and a position check may need commands the journey runner does not have yet.
• The only API journey requires an assembler build with the API explorer. A journey against the isolated docs/ site would add the Elasticsearch spec generation, about two minutes locally, to every run, unless it uses a small spec such as cloud.json. /api/doc/cloud/operation/operation-add-role-assignments has an overflowing curl sample with the note.