Implement NewTaskButton with styling and tests

This commit introduces the NewTaskButton widget, designed to allow users to create new tasks. The button follows specific design guidelines, including dimensions, colors, and layout.

Key Features:

Button with a set height and adaptive width.
Icon and text layout within the button.
Styling for background, border, and corner radius.
Associated tests to ensure the button's functionality and appearance.

lib/views/new_task_button.dart

@@ -0,0 +1,47 @@
+import 'package:flutter/material.dart';
+
+class NewTaskButton extends StatelessWidget {
+  final VoidCallback onPressed;
+
+  const NewTaskButton({Key? key, required this.onPressed}) : super(key: key);
+
+  @override
+  Widget build(BuildContext context) {
+    // Determine the width of the TaskView
+    double taskViewWidth = MediaQuery.of(context).size.width;
+    double buttonWidth = taskViewWidth - 20;
+    if (buttonWidth > 260) {
+      buttonWidth = 260;
+    }
+
+    return ElevatedButton(
+      onPressed: onPressed,
+      style: ButtonStyle(
+        // Set the button's background color
+        backgroundColor: MaterialStateProperty.all<Color>(Colors.white),
+        // Set the button's edge
+        side: MaterialStateProperty.all<BorderSide>(
+            const BorderSide(color: Colors.black, width: 0.5)),
+        // Set the button's shape with rounded corners
+        shape: MaterialStateProperty.all<RoundedRectangleBorder>(
+          RoundedRectangleBorder(
+            borderRadius: BorderRadius.circular(8.0),
+          ),
+        ),
+      ),
+      child: Container(
+        width: buttonWidth,
+        height: 50,
+        child: Row(
+          children: [
+            // Black plus icon
+            Icon(Icons.add, color: Colors.black),
+            SizedBox(width: 8),
+            // "New Task" label
+            Text('New Task', style: TextStyle(color: Colors.black)),
+          ],
+        ),
+      ),
+    );
+  }
+}

test/new_task_button_test.dart

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+import 'package:auto_gpt_flutter_client/views/new_task_button.dart';
+import 'package:flutter/material.dart';
+import 'package:flutter_test/flutter_test.dart';
+
+void main() {
+  testWidgets('NewTaskButton triggers callback on press',
+      (WidgetTester tester) async {
+    bool wasPressed = false;
+
+    // Build our widget.
+    await tester.pumpWidget(MaterialApp(
+      home: Scaffold(
+        body: NewTaskButton(onPressed: () => wasPressed = true),
+      ),
+    ));
+
+    // Verify if the button with the text 'New Task' is displayed.
+    expect(find.text('New Task'), findsOneWidget);
+
+    // Tap the button and verify if the onPressed callback is triggered.
+    await tester.tap(find.byType(ElevatedButton));
+    expect(wasPressed, true);
+  });
+}