feat: add exists method to sandbox sources for existence checks

- Implemented the `exists` method in `SandboxFileSource` and its subclasses to verify the availability of sandbox sources.
- Updated `SandboxFileService` to utilize the new `exists` method for improved error handling when listing files and downloading files.
- Removed the previous check for storage existence in `archive_source.py` and replaced it with the new method.

api/services/sandbox/sandbox_file_service.py

@@ -20,6 +20,12 @@ class SandboxFileService:
             cache_key_prefix="sandbox_files",
         )
 
+    @classmethod
+    def exists(cls, *, tenant_id: str, sandbox_id: str) -> bool:
+        """Check if the sandbox source exists and is available."""
+        browser = SandboxFileBrowser(tenant_id=tenant_id, sandbox_id=sandbox_id)
+        return browser.exists()
+
     @classmethod
     def list_files(
         cls,
@@ -30,9 +36,13 @@ class SandboxFileService:
         recursive: bool = False,
     ) -> list[SandboxFileNode]:
         browser = SandboxFileBrowser(tenant_id=tenant_id, sandbox_id=sandbox_id)
+        if not browser.exists():
+            return []
         return browser.list_files(path=path, recursive=recursive)
 
     @classmethod
     def download_file(cls, *, tenant_id: str, sandbox_id: str, path: str) -> SandboxFileDownloadTicket:
         browser = SandboxFileBrowser(tenant_id=tenant_id, sandbox_id=sandbox_id)
+        if not browser.exists():
+            raise ValueError("Sandbox source not found")
         return browser.download_file(path=path)