add tests for proxy

test/flutter_system_proxy_test.dart

@@ -1,20 +1,106 @@
+import 'dart:io';
+
 import 'package:flutter/services.dart';
 import 'package:flutter_test/flutter_test.dart';
+import 'package:flutter_system_proxy/flutter_system_proxy.dart';
 
 void main() {
   const MethodChannel channel = MethodChannel('flutter_system_proxy');
 
   TestWidgetsFlutterBinding.ensureInitialized();
 
-  setUp(() {
-    channel.setMockMethodCallHandler((MethodCall methodCall) async {
-      return '42';
-    });
-  });
-
   tearDown(() {
     channel.setMockMethodCallHandler(null);
   });
 
-  test('getSystemProxy', () async {});
+  group('getSystemProxy', () {
+    setUp(() {
+      channel.setMockMethodCallHandler((MethodCall methodCall) async {
+        return '42';
+      });
+    });
+
+    test('getSystemProxy', () async {});
+  });
+
+  group('findProxyFromEnvironment', () {
+    test('returns PROXY string when host and port are valid', () async {
+      channel.setMockMethodCallHandler((MethodCall methodCall) async {
+        return <String, dynamic>{'host': '127.0.0.1', 'port': '8080'};
+      });
+
+      final result =
+          await FlutterSystemProxy.findProxyFromEnvironment('https://example.com/v1:drop');
+
+      expect(result, 'PROXY 127.0.0.1:8080');
+    });
+
+    test('falls back to HttpClient.findProxyFromEnvironment when host is empty',
+        () async {
+      channel.setMockMethodCallHandler((MethodCall methodCall) async {
+        return <String, dynamic>{'host': '', 'port': '8080'};
+      });
+
+      final result =
+          await FlutterSystemProxy.findProxyFromEnvironment('https://example.com');
+
+      expect(result,
+          HttpClient.findProxyFromEnvironment(Uri.parse('https://example.com')));
+    });
+
+    test('falls back to HttpClient.findProxyFromEnvironment when host is null',
+        () async {
+      channel.setMockMethodCallHandler((MethodCall methodCall) async {
+        return <String, dynamic>{'host': null, 'port': '8080'};
+      });
+
+      final result =
+          await FlutterSystemProxy.findProxyFromEnvironment('https://example.com');
+
+      expect(result,
+          HttpClient.findProxyFromEnvironment(Uri.parse('https://example.com')));
+    });
+
+    test(
+        'falls back to HttpClient.findProxyFromEnvironment when port is invalid',
+        () async {
+      channel.setMockMethodCallHandler((MethodCall methodCall) async {
+        return <String, dynamic>{'host': '127.0.0.1', 'port': 'not-a-port'};
+      });
+
+      final result =
+          await FlutterSystemProxy.findProxyFromEnvironment('https://example.com');
+
+      expect(result,
+          HttpClient.findProxyFromEnvironment(Uri.parse('https://example.com')));
+    });
+
+    test(
+        'falls back to HttpClient.findProxyFromEnvironment when port is out of range',
+        () async {
+      channel.setMockMethodCallHandler((MethodCall methodCall) async {
+        return <String, dynamic>{'host': '127.0.0.1', 'port': '70000'};
+      });
+
+      final result =
+          await FlutterSystemProxy.findProxyFromEnvironment('https://example.com');
+
+      expect(result,
+          HttpClient.findProxyFromEnvironment(Uri.parse('https://example.com')));
+    });
+
+    test(
+        'falls back to HttpClient.findProxyFromEnvironment when port is out of range',
+        () async {
+      channel.setMockMethodCallHandler((MethodCall methodCall) async {
+        return <String, dynamic>{'host': '127.0.0.1', 'port': '70000'};
+      });
+
+      final result =
+          await FlutterSystemProxy.findProxyFromEnvironment('https://example.com/v1:drop');
+
+      expect(result,
+          HttpClient.findProxyFromEnvironment(Uri.parse('https://example.com/v1:drop')));
+    });
+  });
 }