fix(health): suppress 404 when health monitor disabled + fix audit test assertion

- Sidebar.svelte: defer healthStore.refresh() until feature flags confirm
  health_monitor is enabled; skip entirely when disabled
- health.js: remove throw error from catch block — log and return null
  instead, preventing 'Uncaught (in promise)' on expected 404
- test_report_audit_immutability.py: fix mock assertions —
  audit_service uses logger.reason/reflect/explore, not logger.info
- HealthStore and Sidebar now produce zero network noise when
  FEATURES__HEALTH_MONITOR=false

frontend/src/components/git/CommitModal.svelte

@@ -47,6 +47,8 @@
             // postApi returns the JSON data directly or throws an error
             const data = await api.postApi(
                 `/git/repositories/${encodeURIComponent(String(dashboardId))}/generate-message${envId ? `?env_id=${encodeURIComponent(String(envId))}` : ""}`,
+                undefined,
+                { suppressToast: true },
             );
             message = data.message;
             toast($t.git?.commit_message_generated, "success");