feat: Remove last accessed at default factory (#4795)

Remove default factory on file blocks

letta/schemas/block.py

@@ -93,7 +93,7 @@ class FileBlock(Block):
     source_id: str = Field(..., description="Unique identifier of the source.")
     is_open: bool = Field(..., description="True if the agent currently has the file open.")
     last_accessed_at: Optional[datetime] = Field(
-        default_factory=datetime.utcnow,
+        None,
         description="UTC timestamp of the agent’s most recent access to this file. Any operations from the open, close, or search tools will update this field.",
     )