[jnigen] Respect top-type nullability of type arguments

[HosseinYousefi]

Closes #1906.

Keep the original nullability of a type argument T extends Foo, so that even if the type provided is not explicitly identified as nullable or non-nullable (due to a missing annotation or being a kotlin wildcard), it's still generated with regards to the constraints of the top-type Foo.

[github-actions]

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

[coveralls]

coverage: 80.948%. first build
when pulling 63966a3 on kotlin-top-type-nullability
into 817c76e on main.

[liamappelbe]

This allTypeParams stuff feels like it should be factored out into a little helper class or something. You have little bits of logic for dealing with allTypeParams indexes spread throughout this function. Better to consolidate that logic if possible.

[HosseinYousefi]

Good suggestion, rewriting the logic made the code much better!