Note

Medium Risk
Changes connection initialization behavior around protocol negotiation and maintenance-notification setup; could affect clients that request RESP3 but connect to servers/proxies without HELLO support.

Overview
Prevents initConn from attempting the CLIENT MAINT_NOTIFICATIONS handshake when HELLO fails and the connection effectively falls back to RESP2, even if Options.Protocol requested RESP3.

If maintnotifications were explicitly enabled (ModeEnabled) with Protocol: 3, initConn now fails fast with a clear RESP3/HELLO-related error; for auto modes it silently disables maintnotifications (updates MaintNotificationsConfig.Mode to ModeDisabled and tears down related upgrades) and continues.

Adds a mock RESP2 server and new regression tests to assert no CLIENT MAINT_NOTIFICATIONS command is sent on HELLO fallback, and to validate the differing ModeEnabled vs ModeAuto outcomes.

^{Reviewed by Cursor Bugbot for commit b778286. Bugbot is set up for automated code reviews on this repo. Configure here.}