feat: Reorder create_time and mime_type to ArtifactVersion

Before: http://sponge2/ba05f9ac-c13d-43b6-bb8a-3e1b029cc705(failed)
After: http://sponge2/a623ba76-62c1-4d17-b4b6-22044333a801
PiperOrigin-RevId: 819896989

src/google/adk/artifacts/base_artifact_service.py

@@ -15,6 +15,7 @@ from __future__ import annotations
 
 from abc import ABC
 from abc import abstractmethod
+from datetime import datetime
 from typing import Any
 from typing import Optional
 
@@ -32,6 +33,10 @@ class ArtifactVersion(BaseModel):
   """The canonical URI of the artifact version."""
   custom_metadata: dict[str, Any] = Field(default_factory=dict)
   """A dictionary of custom metadata associated with the artifact version."""
+  create_time: float = Field(default_factory=lambda: datetime.now().timestamp())
+  """The creation time of the artifact version."""
+  mime_type: Optional[str] = None
+  """The MIME type of the artifact version."""
 
 
 class BaseArtifactService(ABC):