Make metadata_file query parameter mandatory

It doesn't work without the parameter and results in
```
  File "/Users/mvandenb/src/galaxy/lib/galaxy/webapps/galaxy/api/datasets.py", line 264, in get_metadata_file
    metadata_file_path, headers = self.service.get_metadata_file(trans, history_content_id, metadata_file)
  File "/Users/mvandenb/src/galaxy/lib/galaxy/webapps/galaxy/services/datasets.py", line 448, in get_metadata_file
    file_ext = hda.metadata.spec.get(metadata_file).get("file_ext", metadata_file)
AttributeError: 'NoneType' object has no attribute 'get'
```

lib/galaxy/webapps/galaxy/api/datasets.py

@@ -256,12 +256,12 @@ class FastAPIDatasets:
         trans=DependsOnTrans,
         history_id: EncodedDatabaseIdField = HistoryIDPathParam,
         history_content_id: EncodedDatabaseIdField = DatasetIDPathParam,
-        metadata_file: Optional[str] = Query(
-            default=None,
+        metadata_name: str = Query(
+            ...,
             description="The name of the metadata file to retrieve.",
         ),
     ):
-        metadata_file_path, headers = self.service.get_metadata_file(trans, history_content_id, metadata_file)
+        metadata_file_path, headers = self.service.get_metadata_file(trans, history_content_id, metadata_name)
         return FileResponse(path=cast(str, metadata_file_path), headers=headers)
 
     @router.get(
@@ -446,13 +446,13 @@ class DatasetsController(BaseGalaxyAPIController):
         return self.service.get_content_as_text(trans, dataset_id)
 
     @web.expose_api_raw_anonymous_and_sessionless
-    def get_metadata_file(self, trans, history_content_id, history_id, metadata_file=None, **kwd):
+    def get_metadata_file(self, trans, history_content_id, history_id, metadata_name, **kwd):
         """
         GET /api/histories/{history_id}/contents/{history_content_id}/metadata_file
         """
         # TODO: remove open_file parameter when deleting this legacy endpoint
         metadata_file, headers = self.service.get_metadata_file(
-            trans, history_content_id, metadata_file, open_file=True
+            trans, history_content_id, metadata_name, open_file=True
         )
         trans.response.headers.update(headers)
         return metadata_file

lib/galaxy/webapps/galaxy/services/datasets.py

@@ -434,7 +434,7 @@ class DatasetsService(ServiceBase, UsesVisualizationMixin):
         self,
         trans: ProvidesHistoryContext,
         history_content_id: EncodedDatabaseIdField,
-        metadata_file: Optional[str] = None,
+        metadata_name: str,
         open_file: bool = False,
     ):
         """
@@ -445,12 +445,12 @@ class DatasetsService(ServiceBase, UsesVisualizationMixin):
         """
         decoded_content_id = self.decode_id(history_content_id)
         hda = self.hda_manager.get_accessible(decoded_content_id, trans.user)
-        file_ext = hda.metadata.spec.get(metadata_file).get("file_ext", metadata_file)
+        file_ext = hda.metadata.spec.get(metadata_name).get("file_ext", metadata_name)
         fname = "".join(c in util.FILENAME_VALID_CHARS and c or "_" for c in hda.name)[0:150]
         headers = {}
         headers["Content-Type"] = "application/octet-stream"
         headers["Content-Disposition"] = f'attachment; filename="Galaxy{hda.hid}-[{fname}].{file_ext}"'
-        file_path = hda.metadata.get(metadata_file).file_name
+        file_path = hda.metadata.get(metadata_name).file_name
         if open_file:
             return open(file_path, "rb"), headers
         return file_path, headers