[jnigen] Add regression tests for native/#693 and clean up TODOs

[HosseinYousefi]

Closes #693. This issue appears to be fixed by one of the other changes already, added a specific regression test for it and removed the related TODOs.

[github-actions]

PR Health

Breaking changes ✔️

Package	Change	Current Version	New Version	Needed Version	Looking good?

API leaks ✔️

The following packages contain symbols visible in the public API, but not exported by the library. Export these symbols or remove them from your publicly visible API.

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License Headers ✔️

// Copyright (c) 2025, 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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no missing headers

All source files should start with a license header.

Unrelated files missing license headers

Files
pkgs/jni/lib/src/third_party/generated_bindings.dart
pkgs/objective_c/lib/src/ns_input_stream.dart

[coveralls]

coverage: 84.5% (+2.8%) from 81.696%
when pulling 5141e4c on fix-693
into 6db256e on main.