Remove AgentRuntime default max ttl

[hzxuzhonghu]

What would you like to be added:

As agent itself maybe long running, like if user want to host openclaw within agentcube agentRuntime, it cannot be deleted after 8 hour.

[hzxuzhonghu]

cc @YaoZengzeng @acsoto

[acsoto]

AgentRuntime.spec.maxSessionDuration currently defaults to 8h, and workload-manager always converts it into Sandbox.Spec.Lifecycle.ShutdownTime, so even an active long-running AgentRuntime can be deleted after 8 hours. For AgentRuntime, I think this should be opt-in; sessionTimeout can still handle idle cleanup. CodeInterpreter can keep the current default max duration.

I can send a PR to make AgentRuntime maxSessionDuration optional/no-default and only set ShutdownTime when it is explicitly configured.

[hzxuzhonghu]

Yeah totally match

[acsoto]

/assign