TestConnectServer reply carries no engine discriminator #3244

Spun out of the #3145 close-out (PR #3149).

Current state: the TestConnectServer reply carries no engine discriminator. Today that is why the two probe-fed dialog render sites cannot be reached with a PostgreSQL payload at all, so nothing wrong renders there; #3149's call-site audit verified this and left them alone.

Why it matters: it is a latent trap. The moment test-connect learns to probe a PostgreSQL target, its reply cannot say which version vocabulary its number belongs to, and 17 is a real major in both engines, so a consumer guessing the engine would be silently believed. That exact failure mode is why MonitoredEngineVersion.DescribeEngineVersion takes the two majors as separate parameters.

Fix shape: add the engine kind (and a PostgreSQL major, populated on that arm) to the TestConnectServer reply, and feed the shared engine-facts reader that #3149 gave the probe-fed dialogs. Do this as part of, or as a prerequisite for, whatever change first teaches test-connect about PostgreSQL targets.