fix(ui): use describeApiError across remaining error-surface sites

Extends the previous describeApiError rollout to the rest of the UI.
Two symptom classes covered:

 - Bare e.message / err.message in toast descriptions would render
   "undefined" on Spring error bodies (plain objects without a proper
   Error prototype). Affected: OidcConfigPage (save/test/delete),
   ClaimMappingRulesModal (save + test), AgentHealth (dismiss),
   RouteControlBar (route action + replay).

 - Inline {String(error)} on load-failure banners would render
   "[object Object]". Affected: InboxPage, RulesListPage, SilencesPage,
   OutboundConnectionsPage.

Not touched: auth-store, AppsTab, UsersTab — they already guard with
`e instanceof Error` and fall back to a static string; replacing the
fallback with describeApiError would be a behavioral change best
evaluated separately.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

ui/src/pages/AgentHealth/AgentHealth.tsx

@@ -23,6 +23,7 @@ import type { ConfigUpdateResponse } from '../../api/queries/commands';
 import type { AgentInstance } from '../../api/types';
 import { timeAgo } from '../../utils/format-utils';
 import { formatUptime, mapLogLevel, eventSeverity, eventIcon } from '../../utils/agent-utils';
+import { describeApiError } from '../../api/errors';
 
 // ── Helpers ──────────────────────────────────────────────────────────────────
 
@@ -506,7 +507,7 @@ export default function AgentHealth() {
                   navigate('/runtime');
                 },
                 onError: (err) => {
-                  toast({ title: 'Dismiss failed', description: err.message, variant: 'error', duration: 86_400_000 });
+                  toast({ title: 'Dismiss failed', description: describeApiError(err), variant: 'error', duration: 86_400_000 });
                 },
               });
             }}