clusterdiscovery: fix login-hint tests for squashed login-server list

d471460 squashed the advertised login-server list and entire auth use hint out of renderLoginHint (until the multi-login UX is ready) but left two tests asserting the old richer message, breaking test-core CI: TestRenderLoginHint and TestResolve_NoEligibleContextReturnsLoginHint.

Update both to assert the new message (and that the URLs / auth-use hint are intentionally absent), and refresh the stale RenderLoginHint doc comment that still claimed "one indented URL per line".