feat(extension-health): cache results, force refresh, treat lookup errors as transient

Cache health results after each check, add a force flag to bypass the cache when the download picker opens or a service is selected, guard stale concurrent results via a per-extension request serial, and treat DNS lookup failures as indeterminate/transient.

lib/widgets/download_service_picker.dart

@@ -1,3 +1,4 @@
+import 'dart:async';
 import 'dart:io';
 import 'package:flutter/material.dart';
 import 'package:flutter_riverpod/flutter_riverpod.dart';
@@ -74,7 +75,9 @@ class _DownloadServicePickerState extends ConsumerState<DownloadServicePicker> {
     super.initState();
     WidgetsBinding.instance.addPostFrameCallback((_) {
       if (!mounted) return;
-      ref.read(extensionProvider.notifier).refreshEnabledExtensionHealth();
+      ref
+          .read(extensionProvider.notifier)
+          .refreshEnabledExtensionHealth(force: true);
     });
     final downloadExtensions = _downloadExtensions();
     final recommended = widget.recommendedService;
@@ -102,6 +105,17 @@ class _DownloadServicePickerState extends ConsumerState<DownloadServicePicker> {
     return const [];
   }
 
+  void _selectService(Extension extension) {
+    setState(() => _selectedService = extension.id);
+    if (extension.hasServiceHealth) {
+      unawaited(
+        ref
+            .read(extensionProvider.notifier)
+            .checkExtensionHealth(extension.id, force: true),
+      );
+    }
+  }
+
   @override
   Widget build(BuildContext context) {
     final colorScheme = Theme.of(context).colorScheme;
@@ -166,8 +180,7 @@ class _DownloadServicePickerState extends ConsumerState<DownloadServicePicker> {
                                 ? extensionState.healthStatuses[ext.id]?.status
                                 : null,
                             isSelected: _selectedService == ext.id,
-                            onTap: () =>
-                                setState(() => _selectedService = ext.id),
+                            onTap: () => _selectService(ext),
                             iconPath: ext.iconPath,
                           ),
                       ],