RTL InputDecoration fix

[justinmc]

It looks like the wrong padding was being used in the layout logic in RTL mode when using a prefixIcon. This fixes it.

This bug seems to have been in the codebase for 5 years. However, it doesn't manifest itself unless 1. RTL 2. prefixIcon and 3. asymmetric left/right contentPadding, so that's probably why it wasn't reported until now.

Before
After

Fixes #129591

Code to reproduce

import 'package:flutter/material.dart';

/// Flutter code sample for [InputDecoration].

void main() => runApp(const InputDecorationExampleApp());

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

  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      home: Scaffold(
        appBar: AppBar(title: const Text('InputDecoration Sample')),
        body: const Padding(
          padding: EdgeInsets.all(8),
          child: Column(
            children: [
              InputDecorationExample(),
              SizedBox(height: 8),
              Directionality(
                textDirection: TextDirection.rtl,
                child: InputDecorationExample(),
              ),
            ],
          ),
        ),
      ),
    );
  }
}

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

  @override
  Widget build(BuildContext context) {
    return const TextField(
      decoration: InputDecoration(
        contentPadding: EdgeInsetsDirectional.only(end: 24),
        prefixIcon: Icon(Icons.search),
      ),
    );
  }
}