Fix DropdownMenu isCollapsed decoration does not Reduce height

[bleroux]

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:

---

Before this PR and #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 #153089, the height can be reduce with InputDecoration.suffixIconConstraints but it results in the icon being misaligned:

After this PR:

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

Code sample

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 https://github.com/flutter/flutter/pull/153089.
              suffixIconConstraints: BoxConstraints(minHeight: 24, maxHeight: 24),
              filled: true,
            ),
          ),
        ),
      ),
    );
  }
}

Related Issue

Fixes DropdownMenu inputDecoration isCollapsed property does not reduce vertical spacing

Tests

Adds 2 tests.

[QuncCccccc]

LGTM:) Thanks for the improvement!