consoleproxy.session.timeout is honoured for noVNC sessions but not legacy VNC/RDP/AJAX viewers

Start by reading ConsoleProxy.VIEWER_LINGER_SECONDS, ConsoleProxy.sessionTimeoutMillis, and isFrontEndAlive() in ConsoleProxyVncClient, ConsoleProxyRdpClient, and ConsoleProxyNoVncClient. Reproduce the issue with consoleproxy.session.timeout set above 180000 ms, then verify that legacy VNC, RDP, AJAX, and noVNC sessions all use the configured idle timeout rather than a separate 180-second threshold.

problem

consoleproxy.session.timeout is now honoured for noVNC console sessions (#12810, PR #13058), but VNC/RDP/AJAX-based console viewers still rely on a separate, hardcoded idle threshold: ConsoleProxy.VIEWER_LINGER_SECONDS (180 seconds), used by isFrontEndAlive() in ConsoleProxyVncClient, ConsoleProxyRdpClient, and ConsoleProxyNoVncClient.

This means the effective idle timeout for a console session now depends on which viewer/hypervisor path served it:

• noVNC sessions: idle timeout follows the configured consoleproxy.session.timeout (both via ConsoleProxyGCThread and, since PR #13058, the WebSocket session's own idle timeout).
• Legacy VNC/RDP/AJAX sessions: idle timeout is still fixed at 180 seconds regardless of the consoleproxy.session.timeout setting.

An admin who sets consoleproxy.session.timeout to, say, 30 minutes to keep long-idle sessions open will still see legacy VNC/RDP/AJAX sessions get dropped after 3 minutes — an inconsistency that's confusing and undocumented.

versions

ACS 4.20+ (any branch carrying the PR #13058 / equivalent noVNC timeout fix)

The steps to reproduce the bug

1. Set the global setting consoleproxy.session.timeout to a value well above 180000 ms (e.g. 1800000 ms / 30 minutes).
2. Open a console session that uses the legacy VNC/RDP/AJAX path (e.g. a hypervisor or console mode that doesn't route through noVNC).
3. Leave the session idle.
4. Observe the session is torn down after ~180 seconds, not after the configured consoleproxy.session.timeout.
5. Compare against a noVNC console session under the same setting, which correctly honours the configured value.

What to do about it?

Derive VIEWER_LINGER_SECONDS from the same configured consoleproxy.session.timeout value (ConsoleProxy.sessionTimeoutMillis) instead of keeping it as an independent hardcoded constant, so all console viewer types (noVNC, VNC, RDP, AJAX) honour one single, consistently-configured idle timeout.

This was flagged during review of PR #13058 (https://github.com/apache/cloudstack/pull/13058#discussion_r3309508273) but deliberately left out of that PR's scope, since it's a noVNC-focused backport and touching isFrontEndAlive() for the legacy viewer types is a broader behavioural change that deserves its own review/testing pass.