feat(gateway): harden failover and payload handling

Retry pre-response transport failures across candidates with an explicit stop policy, and propagate end-to-end timing into usage records and UI diagnostics.

Remove legacy body, import, cookie, PII, and tunnel replay caps while preserving optional operator-configured gateway limits.

frontend/src/features/providers/components/__tests__/ProviderDetailDrawer.loading.spec.ts

@@ -54,4 +54,8 @@ describe('ProviderDetailDrawer loading priorities', () => {
     expect(source).toContain('v-if="open && batchAssignDialogOpen && provider"')
     expect(source).toContain('v-if="open && failoverRulesDialogOpen"')
   })
+
+  it('marks a transport-error stop policy as a configured failover rule', () => {
+    expect(source).toContain('rules.stop_on_transport_errors === true')
+  })
 })