[jnigen] Kotlin void suspend funcs now return Future<void> in Dart

[liamappelbe]

Unit is a singleton sentinel used by Kotlin when translating suspend funcs that return void into Java methods. It's needed because Java doesn't support void as a generic type param, so various internal objects like Continuation<T> are represented as Continuation<Unit> instead of Continuation<void>.

This PR special cases Unit in the return type of a suspend function to code gen as Future<void> instead of Future<JObject>. Same for interfaces with void suspend functions.

Ordinary functions that return void are translated directly into Java methods that return void, so already return void in Dart land. Also, Unit is a real object, so it could conceivably be passed as a param to a method etc. So this special casing should only be applied to the return type of suspend functions.

Fixes #2876

[github-actions]

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// Copyright (c) 2026, the Dart project authors. Please see the AUTHORS file
// for details. All rights reserved. Use of this source code is governed by a
// BSD-style license that can be found in the LICENSE file.

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pkgs/hooks_runner/test_data/download_assets/hook/build.dart
pkgs/jni/test/debug_release_test.dart
pkgs/objective_c/example/command_line/lib/main.dart
pkgs/objective_c/lib/src/ns_input_stream.dart

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Breaking changes ⚠️

Package	Change	Current Version	New Version	Needed Version	Looking good?
jni	Breaking	0.15.2	0.15.3-wip	0.16.0 Got "0.15.3-wip" expected >= "0.16.0" (breaking changes)	⚠️

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[coveralls]

coverage: 81.234% (-0.01%) from 81.248%
when pulling be44e7f on jnigen_unit_type
into 3495c21 on main.