[image_picker] Remove use of Pigeon's Dart test generator (#10320)

Updates `image_picker_ios` to use the same test structure that `image_picker_android` already uses:
- Removes the use of Pigeon's Dart test generator in favor of DI'ing an implementation of the actual generated Dart API.
- Removes the use of the old method channel logger (which was adapted in place during the Pigeon conversion to minimize test changes in the PR that changed the implementation; the fact that it was still being used was tech debt), in favor of structured storage of passed arguments that can be queried directly. This makes tests much shorter and easier to understand and maintain, because if we want to check one parameter in a targeted test, we don't have to assert the entire argument dictionary.
- Adds some missing test functionality (e.g., we didn't have tests of any of the non-null return value handling), and removes some duplicate testing (e.g., every possible combination of null and non-null width, hight, and image quality in on giant test).

Also does minor cleanup of `image_picker_android`; this conversion had already been done there, but it was still generating an unused `dartTestOut`.

Part of flutter/flutter#159886

## Pre-Review Checklist

[^1]: Regular contributors who have demonstrated familiarity with the repository guidelines only need to comment if the PR is not auto-exempted by repo tooling.

Author: stuartmorgan-g

packages/image_picker/image_picker_android/pigeons/messages.dart

@@ -7,7 +7,6 @@ import 'package:pigeon/pigeon.dart';
 @ConfigurePigeon(
   PigeonOptions(
     dartOut: 'lib/src/messages.g.dart',
-    dartTestOut: 'test/test_api.g.dart',
     javaOut:
         'android/src/main/java/io/flutter/plugins/imagepicker/Messages.java',
     javaOptions: JavaOptions(package: 'io.flutter.plugins.imagepicker'),
@@ -94,7 +93,7 @@ class CacheRetrievalResult {
   final List<String> paths;
 }
 
-@HostApi(dartHostTestHandler: 'TestHostImagePickerApi')
+@HostApi()
 abstract class ImagePickerApi {
   /// Selects images and returns their paths.
   @TaskQueue(type: TaskQueueType.serialBackgroundThread)

packages/image_picker/image_picker_android/test/test_api.g.dart

@@ -3,6 +3,7 @@
 // found in the LICENSE file.
 import 'dart:async';
 
+import 'package:flutter/foundation.dart' show visibleForTesting;
 import 'package:image_picker_platform_interface/image_picker_platform_interface.dart';
 
 import 'src/messages.g.dart';
@@ -43,7 +44,11 @@ SourceCamera _convertCamera(CameraDevice camera) {
 
 /// An implementation of [ImagePickerPlatform] for iOS.
 class ImagePickerIOS extends ImagePickerPlatform {
-  final ImagePickerApi _hostApi = ImagePickerApi();
+  /// Creates a new plugin implementation instance.
+  ImagePickerIOS({@visibleForTesting ImagePickerApi? api})
+    : _hostApi = api ?? ImagePickerApi();
+
+  final ImagePickerApi _hostApi;
 
   /// Registers this class as the default platform implementation.
   static void registerWith() {

packages/image_picker/image_picker_ios/lib/image_picker_ios.dart

@@ -7,7 +7,6 @@ import 'package:pigeon/pigeon.dart';
 @ConfigurePigeon(
   PigeonOptions(
     dartOut: 'lib/src/messages.g.dart',
-    dartTestOut: 'test/test_api.g.dart',
     objcHeaderOut:
         'ios/image_picker_ios/Sources/image_picker_ios/include/image_picker_ios/messages.g.h',
     objcSourceOut: 'ios/image_picker_ios/Sources/image_picker_ios/messages.g.m',
@@ -52,7 +51,7 @@ class SourceSpecification {
   SourceCamera camera;
 }
 
-@HostApi(dartHostTestHandler: 'TestHostImagePickerApi')
+@HostApi()
 abstract class ImagePickerApi {
   @async
   @ObjCSelector('pickImageWithSource:maxSize:quality:fullMetadata:')