test: rename SearchSpace -> Workspace across tests + fixtures (Phase 2 Wave F)

Apply the same rename to surfsense_backend/tests: workspace_id fields/vars,
Workspace* classes/schemas, table name searchspaces -> workspaces in raw SQL,
and the API URL spellings -> /workspaces. Preserves the carve-out wire literals
tests assert (Celery task names, OTel key "search_space.id").

surfsense_backend/tests/unit/middleware/test_knowledge_tree.py

@@ -101,7 +101,7 @@ class TestFormatTreeRendering:
         docs = [_Row(**spec) for spec in doc_specs]
 
         mw = KnowledgeTreeMiddleware(
-            search_space_id=1,
+            workspace_id=1,
             filesystem_mode=None,  # type: ignore[arg-type]
         )
         return mw._format_tree(index, docs)