refactor: replace enum sandboxType to string sandboxType

[ShenJunkun]

Description

This PR addresses #65 by replacing the rigid SandboxType enum with a string-based identifier. This change allows users to register custom sandbox types beyond the built-in options, supporting diverse scenarios like domain-specific images or custom security policies.

Changes

• Replaced SandboxType enum with String in @RegisterSandbox and registry logic.
• Defined SandboxType constants to maintain backward compatibility.
• Updated the identification key to (userId, sessionId, sandboxType).

Checklist

• My code follows the code style of this project.
• I have performed a self-review of my own code.
• All new and existing tests passed.

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