Use dataclass to initialize default values

lib/galaxy/files/sources/__init__.py

@@ -1,4 +1,5 @@
 import abc
+from dataclasses import dataclass, field
 import os
 import time
 from enum import Enum
@@ -88,6 +89,7 @@ class FilesSourceProperties(TypedDict):
     browsable: NotRequired[bool]
 
 
+@dataclass
 class FilesSourceOptions:
     """Options to control behavior of file source operations, such as realize_to, write_from and list."""
 
@@ -100,7 +102,7 @@ class FilesSourceOptions:
     # the HTTPFilesSource passes in additional http_headers through these properties, which
     # are merged with constructor defined http_headers. The interpretation of these properties
     # are filesystem specific.
-    extra_props: Optional[FilesSourceProperties] = {}
+    extra_props: Optional[FilesSourceProperties] = field(default_factory=lambda: {})
 
 
 class EntryData(TypedDict):