Add Tooltip default vertical padding

[bleroux]

Description

This PR add a Tooltip default vertical padding for developper convenience.
see #86170 (comment)

@guidezpl
After investigating various solutions to not rely on vertical padding, I reached the conclusion that setting a default vertical padding is the best candidate :

• Users can easily override the default value as there is already a padding property on Tooltip (without relying on vertical padding we should add a new property and manage edge cases where the user already set the padding property).
• Before rendering, there is no simple way to know when a Text widget will wrap, so adding layout logic to increase Tooltip container height when the Text wraps will be complex and error-prone.

Related Issue

Fixes #86170

Tests

Update 3 tests.

[guidezpl]

Hi! I believe this is a breaking change and won't be able to land before preliminary work.

Also, this PR would benefit from a table with screenshots showing the resulting tooltip under various combinations (mobile/desktop, non-wrapping, wrapping).

[bleroux]

Hey!
I tried to take several screenshots to demonstrate some combinations.

Code sample for screenshots

import 'package:flutter/material.dart';

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

class MyApp extends StatelessWidget {
  const MyApp({super.key, this.dark = false, this.useMaterial3 = true});

  final bool dark;
  final bool useMaterial3;

  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      debugShowCheckedModeBanner: false,
      themeMode: dark ? ThemeMode.dark : ThemeMode.light,
      theme: ThemeData(
        useMaterial3: useMaterial3,
        brightness: Brightness.light,
        colorSchemeSeed: const Color(0xff6750a4),
      ),
      darkTheme: ThemeData(
        useMaterial3: useMaterial3,
        brightness: Brightness.dark,
        colorSchemeSeed: const Color(0xff6750a4),
      ),
      home: const Example(),
    );
  }
}

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

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: const Text('Tooltip Sample'),
      ),
      body: Center(
        child: Column(
          mainAxisAlignment: MainAxisAlignment.spaceEvenly,
          children: <Widget>[
            Column(
              children: <Widget>[
                const SizedBox(height: 5),
                Container(
                  color: Colors.green[100],
                  child: const Tooltip(
                    message: "Non-wrapping Tooltip",
                    triggerMode: TooltipTriggerMode.tap,
                    showDuration: Duration(seconds: 5),
                    preferBelow: false,
                    child: Padding(
                      padding: EdgeInsets.all(8.0),
                      child: Text(
                        'Tap me to display\na non wrapping Tooltip',
                        textAlign: TextAlign.center,
                      ),
                    ),
                  ),
                ),
              ],
            ),
            Column(
              children: <Widget>[
                const SizedBox(height: 5),
                Container(
                  color: Colors.green[100],
                  child: const Tooltip(
                    message: "Wrapping\nTooltip",
                    triggerMode: TooltipTriggerMode.tap,
                    showDuration: Duration(seconds: 5),
                    preferBelow: false,
                    child: Padding(
                      padding: EdgeInsets.all(8.0),
                      child: Text(
                        'Tap me to display\na wrapping Tooltip',
                        textAlign: TextAlign.center,
                      ),
                    ),
                  ),
                ),
              ],
            ),
          ],
        ),
      ),
    );
  }
}

I also updated the PR with a vertical padding of 4.0 instead of 6.0 to make it less risky. The PR passed all customer_testing tests but I don't know if it might be a breaking change for some Google internal testing.

Desktop (Unbuntu) :

Android :

[guidezpl]

LGTM, nice. Let's see what testing says