[Testing] Improve Test Coverage: Add Missing Unit Tests and Enable Integration Tests with Mock Sandbox Client

[xuehuitian45]

Description

The current test suite has the following gaps:

1. Incomplete unit test coverage – Core components lack sufficient unit tests.
2. Integration tests are missing or disabled – Notably, integration tests involving the sandbox module were previously disabled due to extremely slow startup time of real sandbox environments (e.g., Docker-based executors).
3. No mock abstraction for sandbox client – This forces tests to either skip sandbox logic or rely on slow, flaky real environments.

Current Impact

• Low confidence in code changes
• CI pipelines skip critical integration paths
• Hard to validate new features end-to-end

Proposed Solution

1. Introduce a Mock Sandbox Client

2. Re-enable and Expand Integration Tests

3. Improve Unit Test Coverage

4. CI/CD Integration

Benefits

• Fast, reliable CI feedback loop
• Higher confidence code changes
• Easier contribution from external developers
• Foundation for testing advanced features (multi-sandbox, etc.)