A test pins that the proxy serves only verified domains: `getVerifiedDomain`, `primaryDomain`, `removeDomain` #6791

src/core/domains.ts says getVerifiedDomain returns "the only kind the proxy serves"; no test calls it, primaryDomain, listDomains or removeDomain directly.

Done when a new test file proves:

• [ ] an added, unverified host is listed by listDomains but getVerifiedDomain is undefined; after a passing recordCheck it is returned; after a failing check it behaves as the module documents.
• [ ] the lookup is exact: a different case or a trailing dot is handled as the module documents, and a subdomain of a verified host is not served.
• [ ] primaryDomain names the app's primary; removeDomain removes it and getVerifiedDomain is undefined afterwards.

Test only; a bug it finds is fixed in the same pull.