feat: Add method to check agent online status and use it in container handler

[9trocode]

This pull request improves the accuracy of agent online status detection for shared terminal features by introducing a new method to check real-time agent connectivity, both locally and across distributed instances via Redis. The changes ensure that the system more reliably reports whether agents are actually online, rather than relying solely on historical ping data.

Enhancements to agent online status detection:

• Added the IsAgentOnline method to AgentHandler in internal/api/handlers/agent.go, which checks for real-time agent connectivity locally and via Redis, providing a more accurate online status.

Improvements to shared terminal logic:

• Updated the shared terminal status determination in internal/api/handlers/container.go to use the new IsAgentOnline method, falling back to the previous LastPing check only if real-time connectivity is not found.