refactor(box): unify box service lifecycle and local runtime

management

src/langbot/pkg/box/errors.py

@@ -13,5 +13,13 @@ class BoxBackendUnavailableError(BoxError):
     """Raised when no supported container backend is available."""
 
 
+class BoxRuntimeUnavailableError(BoxError):
+    """Raised when the standalone Box Runtime service is unavailable."""
+
+
 class BoxSessionConflictError(BoxError):
     """Raised when an existing session cannot satisfy a new request."""
+
+
+class BoxSessionNotFoundError(BoxError):
+    """Raised when a referenced session does not exist."""