docs(specs): spec the namespace connection test for unsaved remotes

Why

The monolith remote create form offers Test connection before the form is submitted (MVP prototype, matching the shipped Virtual Registry create form), and no AR surface can serve that button: create validates the submitted url statically and never probes, and the connection test is repository-scoped. The S17 spec already records the monolith/S07 divergence and owes the S07 amendment in its Follow-ups. This patch specs POST /api/v1/:slug/connection_test, probing a body-supplied {format, url, credentials?} through the same covers, probe timeout, credential attachment, and sub-500 reachability rule as the repository-scoped test, persisting nothing. The monolith/S07 amendment is not in this MR: it rides the S07 Phase 2 respec, which declares this create-form gap and gates it on an S17 amendment contracting the endpoint, docs(specs): specify remote and virtual kinds i... (!1760 - merged) • Rahul Chanila • 19.4. The S17 Follow-ups bullet now names that carrier.

Acceptance criteria and Resolutions are appended at list end, so no existing number shifts and the AC citations in flight stay valid (AC #83's create-time coverage is fetch-only: up... (#715) • Hayley Swimelar • 19.4 cites AC #83).

Implementation is gated: the plan MR follows this patch, and the code follows the repository-scoped test, whose probe plumbing it reuses (feat(managementapi): serve the upstream connect... (!1632 - merged) • Hayley Swimelar • 19.4).

Test plan

mise run lint:docs and mise run lint:links are green: markdownlint 0 errors, Vale 0 errors, 0 broken fragments.

Related to Namespace connection test: probe submitted URL ... (#812 - closed) • Hayley Swimelar • 19.4.