feat(sandbox): add AppAssetsInitializer and refactor VMFactory to VMBuilder

- Add AppAssetsInitializer to load published app assets into sandbox
- Refactor VMFactory.create() to VMBuilder with builder pattern
- Extract SandboxInitializer base class and DifyCliInitializer
- Simplify SandboxLayer constructor (remove options/environments params)
- Fix circular import in sandbox module by removing eager SandboxBashTool export
- Update SandboxProviderService to return VMBuilder instead of VirtualEnvironment

api/core/sandbox/initializer/base.py

@@ -0,0 +1,8 @@
+from abc import ABC, abstractmethod
+
+from core.virtual_environment.__base.virtual_environment import VirtualEnvironment
+
+
+class SandboxInitializer(ABC):
+    @abstractmethod
+    def initialize(self, env: VirtualEnvironment) -> None: ...