[Feature Request] Add Container Name Conflict Check Before Sandbox Container Creation

[xuehuitian45]

Description

Currently, when the sandbox manager creates a container, it does not check whether a container with the same name already exists. This can lead to:

• Container creation failure with error: Conflict. The container name "/xxx" is already in use
• Silent misbehavior if the existing container is from a previous session (stale state, wrong user/session context)
• Hard-to-debug issues in multi-agent or high-concurrency environments where naming collisions are more likely

To improve robustness and reliability, we should explicitly check for existing containers with the same name before attempting creation, and handle conflicts gracefully.