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/providers/extension_provider.dart

@@ -814,6 +814,7 @@ class ExtensionNotifier extends Notifier<ExtensionState> {
   Completer<void>? _initializationCompleter;
   final Map<String, DateTime> _healthExpiresAt = {};
   final Map<String, Future<ExtensionHealthStatus?>> _healthInFlight = {};
+  final Map<String, int> _healthRequestSerial = {};
 
   @override
   ExtensionState build() {
@@ -825,6 +826,7 @@ class ExtensionNotifier extends Notifier<ExtensionState> {
       _appLifecycleListener = null;
       _healthExpiresAt.clear();
       _healthInFlight.clear();
+      _healthRequestSerial.clear();
     });
     return const ExtensionState();
   }
@@ -954,15 +956,18 @@ class ExtensionNotifier extends Notifier<ExtensionState> {
     }
   }
 
-  void _scheduleExtensionHealthRefresh(List<Extension> extensions) {
+  void _scheduleExtensionHealthRefresh(
+    List<Extension> extensions, {
+    bool force = false,
+  }) {
     for (final ext in extensions) {
       if (!ext.enabled || !ext.hasServiceHealth) continue;
-      unawaited(checkExtensionHealth(ext.id));
+      unawaited(checkExtensionHealth(ext.id, force: force));
     }
   }
 
-  void refreshEnabledExtensionHealth() {
-    _scheduleExtensionHealthRefresh(state.extensions);
+  void refreshEnabledExtensionHealth({bool force = false}) {
+    _scheduleExtensionHealthRefresh(state.extensions, force: force);
   }
 
   Future<ExtensionHealthStatus?> checkExtensionHealth(
@@ -990,17 +995,22 @@ class ExtensionNotifier extends Notifier<ExtensionState> {
       return inFlight;
     }
 
+    final requestSerial = (_healthRequestSerial[extensionId] ?? 0) + 1;
+    _healthRequestSerial[extensionId] = requestSerial;
+
     final future = () async {
       try {
         final result = await PlatformBridge.checkExtensionHealth(extensionId);
         final status = ExtensionHealthStatus.fromJson(result);
-        final updated = Map<String, ExtensionHealthStatus>.of(
-          state.healthStatuses,
-        )..[extensionId] = status;
-        _healthExpiresAt[extensionId] = DateTime.now().add(
-          _extensionHealthCacheTtl,
-        );
-        state = state.copyWith(healthStatuses: updated);
+        if (_healthRequestSerial[extensionId] == requestSerial) {
+          final updated = Map<String, ExtensionHealthStatus>.of(
+            state.healthStatuses,
+          )..[extensionId] = status;
+          _healthExpiresAt[extensionId] = DateTime.now().add(
+            _extensionHealthCacheTtl,
+          );
+          state = state.copyWith(healthStatuses: updated);
+        }
         return status;
       } catch (e) {
         _log.w('Failed to check extension health for $extensionId: $e');
@@ -1010,16 +1020,20 @@ class ExtensionNotifier extends Notifier<ExtensionState> {
           checkedAt: DateTime.now(),
           checks: const [],
         );
-        final updated = Map<String, ExtensionHealthStatus>.of(
-          state.healthStatuses,
-        )..[extensionId] = status;
-        _healthExpiresAt[extensionId] = DateTime.now().add(
-          const Duration(seconds: 20),
-        );
-        state = state.copyWith(healthStatuses: updated);
+        if (_healthRequestSerial[extensionId] == requestSerial) {
+          final updated = Map<String, ExtensionHealthStatus>.of(
+            state.healthStatuses,
+          )..[extensionId] = status;
+          _healthExpiresAt[extensionId] = DateTime.now().add(
+            const Duration(seconds: 20),
+          );
+          state = state.copyWith(healthStatuses: updated);
+        }
         return status;
       } finally {
-        _healthInFlight.remove(extensionId);
+        if (_healthRequestSerial[extensionId] == requestSerial) {
+          _healthInFlight.remove(extensionId);
+        }
       }
     }();
 

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,
                           ),
                       ],