Discriminate history exports response union

The index_exports endpoint returns either JobExportHistoryArchiveListResponse
or ExportTaskListResponse depending on the Accept header. Without a
discriminator Pydantic walks the union left-to-right at serialization and
warns "Expected JobExportHistoryArchiveListResponse" for every task-based
response. Add a callable Discriminator that routes by instance type, matching
the pattern used for the nested export payload union.

Author: mvdbeek

lib/galaxy/webapps/galaxy/api/histories.py

@@ -22,6 +22,10 @@
     Response,
     status,
 )
+from pydantic import (
+    Discriminator,
+    Tag,
+)
 from pydantic.fields import Field
 from pydantic.main import BaseModel
 
@@ -186,6 +190,19 @@ class CreateHistoryFormData(CreateHistoryPayload):
 ]
 
 
+def _index_exports_response_discriminator(value: Any) -> str:
+    return "tasks" if isinstance(value, ExportTaskListResponse) else "jobs"
+
+
+IndexExportsResponse = Annotated[
+    Union[
+        Annotated[JobExportHistoryArchiveListResponse, Tag("jobs")],
+        Annotated[ExportTaskListResponse, Tag("tasks")],
+    ],
+    Discriminator(_index_exports_response_discriminator),
+]
+
+
 @router.cbv
 class FastAPIHistories:
     service: HistoriesService = depends(HistoriesService)
@@ -551,7 +568,7 @@ def index_exports(
         limit: Optional[int] = LimitQueryParam,
         offset: Optional[int] = OffsetQueryParam,
         accept: IndexExportsAcceptHeader = "application/json",
-    ) -> Union[JobExportHistoryArchiveListResponse, ExportTaskListResponse]:
+    ) -> IndexExportsResponse:
         """
         By default the legacy job-based history exports (jeha) are returned.