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

### 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.)