Fix DropdownMenu isCollapsed decoration does not Reduce height (flutter#161427)

## Description

This PR fixes DropdownMenu arrow icon position when
`InputDecoration.isCollapsed` is set to true and
`InputDecoration.suffixConstraints` is set to a smaller value than the
min interactive height.

It makes it possible to use collapsed `DropdownMenu` such as:


![image](https://github.com/user-attachments/assets/145c2a0b-a638-4226-ae29-0e21bb9d4bb0)

_____

Before this PR and flutter#153089,
`InputDecoration.isCollapsed` had no impact on the `DropdownMenu` height
and there was no solution to reduce the height because its minimum
height is enforced by the `IconButton` (arrow down or up) which is part
of the `DropdownMenu`.

Since flutter#153089, the height can be
reduce with `InputDecoration.suffixIconConstraints` but it results in
the icon being misaligned:


![image](https://github.com/user-attachments/assets/2a4d99bc-c26d-454b-8e62-dd8828789ae5)


After this PR:

When `InputDecoration.suffixIconConstraints` is used the icon is
correctly aligned:


![image](https://github.com/user-attachments/assets/145c2a0b-a638-4226-ae29-0e21bb9d4bb0)


<details><summary>Code sample</summary>

```dart
import 'package:flutter/material.dart';

void main() {
  runApp(const MyApp());
}

class MyApp extends StatelessWidget {
  const MyApp({super.key});

  @OverRide
  Widget build(BuildContext context) {
    return const MaterialApp(
      title: 'Flutter Demo',
      home: MyHomePage(title: 'Flutter Demo Home Page'),
    );
  }
}

class MyHomePage extends StatefulWidget {
  const MyHomePage({super.key, required this.title});
  final String title;

  @OverRide
  State<MyHomePage> createState() => _MyHomePageState();
}

class _MyHomePageState extends State<MyHomePage> {
  @OverRide
  Widget build(BuildContext context) {
    return const Scaffold(
      body: Center(
        child: Center(
          child: DropdownMenu<String>(
            dropdownMenuEntries: [
              DropdownMenuEntry(label: 'Item 1', value: '1'),
              DropdownMenuEntry(label: 'Item 2', value: '2'),
            ],
            inputDecorationTheme: InputDecorationTheme(
              contentPadding: EdgeInsets.fromLTRB(5, 0, 5, 0),
              isCollapsed: true,
              // Usable since flutter#153089.
              suffixIconConstraints: BoxConstraints(minHeight: 24, maxHeight: 24),
              filled: true,
            ),
          ),
        ),
      ),
    );
  }
}

``` 

</details> 

## Related Issue

Fixes [DropdownMenu inputDecoration isCollapsed property does not reduce
vertical spacing](flutter#138691)

## Tests

Adds 2 tests.

Author: bleroux

packages/flutter/lib/src/material/dropdown_menu.dart

@@ -965,10 +965,13 @@ class _DropdownMenuState<T> extends State<DropdownMenu<T>> {
       crossAxisUnconstrained: false,
       builder: (BuildContext context, MenuController controller, Widget? child) {
         assert(_initialMenu != null);
+        final bool isCollapsed = widget.inputDecorationTheme?.isCollapsed ?? false;
         final Widget trailingButton = Padding(
-          padding: const EdgeInsets.all(4.0),
+          padding: isCollapsed ? EdgeInsets.zero : const EdgeInsets.all(4.0),
           child: IconButton(
             isSelected: controller.isOpen,
+            constraints: widget.inputDecorationTheme?.suffixIconConstraints,
+            padding: isCollapsed ? EdgeInsets.zero : null,
             icon: widget.trailingIcon ?? const Icon(Icons.arrow_drop_down),
             selectedIcon: widget.selectedTrailingIcon ?? const Icon(Icons.arrow_drop_up),
             onPressed:

packages/flutter/test/material/dropdown_menu_test.dart

@@ -46,6 +46,7 @@ void main() {
     double? menuHeight,
     Widget? leadingIcon,
     Widget? label,
+    InputDecorationTheme? decorationTheme,
   }) {
     return MaterialApp(
       theme: themeData,
@@ -56,6 +57,7 @@ void main() {
           width: width,
           menuHeight: menuHeight,
           dropdownMenuEntries: entries,
+          inputDecorationTheme: decorationTheme,
         ),
       ),
     );
@@ -1172,6 +1174,71 @@ void main() {
     expect(iconButton, findsOneWidget);
   });
 
+  testWidgets('Trailing IconButton height respects InputDecorationTheme.suffixIconConstraints', (
+    WidgetTester tester,
+  ) async {
+    final ThemeData themeData = ThemeData();
+
+    // Default suffix icon constraints.
+    await tester.pumpWidget(buildTest(themeData, menuChildren));
+    await tester.pump();
+
+    final Finder iconButton = find.widgetWithIcon(IconButton, Icons.arrow_drop_down).first;
+    expect(tester.getSize(iconButton), const Size(48, 48));
+
+    // Custom suffix icon constraints.
+    await tester.pumpWidget(
+      buildTest(
+        themeData,
+        menuChildren,
+        decorationTheme: const InputDecorationTheme(
+          suffixIconConstraints: BoxConstraints(minWidth: 66, minHeight: 62),
+        ),
+      ),
+    );
+    await tester.pump();
+
+    expect(tester.getSize(iconButton), const Size(66, 62));
+  });
+
+  testWidgets('InputDecorationTheme.isCollapsed reduces height', (WidgetTester tester) async {
+    final ThemeData themeData = ThemeData();
+
+    // Default height.
+    await tester.pumpWidget(buildTest(themeData, menuChildren));
+    await tester.pump();
+
+    final Finder textField = find.byType(TextField).first;
+    expect(tester.getSize(textField).height, 56);
+
+    // Collapsed height.
+    await tester.pumpWidget(
+      buildTest(
+        themeData,
+        menuChildren,
+        decorationTheme: const InputDecorationTheme(isCollapsed: true),
+      ),
+    );
+    await tester.pump();
+
+    expect(tester.getSize(textField).height, 48); // IconButton min height.
+
+    // Collapsed height with custom suffix icon constraints.
+    await tester.pumpWidget(
+      buildTest(
+        themeData,
+        menuChildren,
+        decorationTheme: const InputDecorationTheme(
+          isCollapsed: true,
+          suffixIconConstraints: BoxConstraints(maxWidth: 24, maxHeight: 24),
+        ),
+      ),
+    );
+    await tester.pump();
+
+    expect(tester.getSize(textField).height, 24);
+  });
+
   testWidgets('Do not crash when resize window during menu opening', (WidgetTester tester) async {
     addTearDown(tester.view.reset);
     final ThemeData themeData = ThemeData();