docs(plans): plan the namespace connection test

Why

The namespace connection test's spec patch merged, and the repo gate requires a plan MR before any implementation MR opens. This plans the endpoint end to end in AR, in two steps: extract the record-free reachability seam from the health monitor, which must probe without moving health columns or the failure counter, then serve POST /api/v1/:slug/connection_test with per-format probe builders over submitted values and its own audit event carrying the normalized URL. The contract is docs(specs): spec the namespace connection test... (!1847 - merged) • Hayley Swimelar • 19.4, and the probe plumbing it reuses landed in feat(managementapi): serve the upstream connect... (!1632 - merged) • Hayley Swimelar • 19.4.

Called out for review: S09's Step 16 moves every management registration onto a binding-required helper, and its route census went stale when the repository-scoped test's registration landed in feat(managementapi): serve the upstream connect... (!1632 - merged) • Hayley Swimelar • 19.4 (32 routes listed, 33 registered, this endpoint is the 34th). Research Findings records how either merge order resolves.

Test plan

Docs-only. markdownlint and Vale report zero errors locally, and docs:lint gates in CI.

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