Add support for disconnectedOnStartup node property with launch prevention

[somiljain2006]

Fixes #1793

Implement the disconnectedOnStartup node property to enforce the offline state and block agent launch

• Add DisconnectedOnStartupProperty with JCasC support
• Mark nodes offline on startup with a custom JCasCOfflineCause
• Prevent agent launch using ComputerListener (preLaunch)
• Ensure nodes remain online when the property is disabled or not defined
• Add tests to verify behavior for enabled, disabled, and default cases

Verified via JenkinsRule tests and manual testing (SSH launcher).

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[somiljain2006]

@timja Can you review this pr?

[timja]

not sure on this, we try minimise this sort of thing. Since its core though it might be needed.

@jetersen any thoughts?