[ffigen] Migrate callbacks to Dart_CObject_kNativePointer

[mertalev]

Description

As per #3265, ports are safer than callbacks as they do not cause a crash when native code tries to reach an isolate that no longer exists. These crashes can occur during hot restart, when background tasks exit, and so on. This PR implements the ObjC side of the proposed changes with the same design as the jnigen fix (#3452). It delivers through a Dart port using Dart_CObject_kNativePointer, so every outcome has an owner. Specifically:

1. If the post is delivered, the per-isolate invoker handles the post, frees the invocation and releases the block
2. If the post is refused, ownership stays with the poster, which cleans up inline
3. If the post is sent but not handled, the VM runs the message's cleanup finalizer

In addition to dedicated unit tests, the changes were tested locally via cupertino_http in an app that previously crashed on hot restart due to dart-lang/http#1894.

Related Issues

Part of #3265

PR Checklist

• I’ve reviewed the contributor guide and applied the relevant portions to this PR.
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• All existing and new tests are passing. I added new tests to check the change I am making.
• The PR is actually solving the issue. PRs that don't solve the issue will be closed. Please be respectful of the maintainers' time. If it's not clear what the issue is, feel free to ask questions on the GitHub issue before submitting a PR.
• I have updated CHANGELOG.md for the relevant packages. (Not needed for small changes such as doc typos).
• I have updated the pubspec package version if necessary.

[github-actions]

PR Health

Breaking changes ⚠️

Package	Change	Current Version	New Version	Needed Version	Looking good?
objective_c	Breaking	9.4.1	9.4.1	10.0.0 Got "9.4.1" expected >= "10.0.0" (breaking changes)	⚠️

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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.

Package	Leaked API symbol	Leaking sources

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Changelog Entry ❗

Package	Changed Files
package:objective_c	pkgs/objective_c/lib/objective_c.dart pkgs/objective_c/lib/src/c_bindings_generated.dart pkgs/objective_c/lib/src/internal.dart pkgs/objective_c/lib/src/objective_c_bindings_generated.dart

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[liamappelbe]

Thanks for the PR @mertalev, but I think I should do this one myself. I've already got a WIP PR for this, and I'm trying to solve the problem not just for listener blocks, but also blocking blocks, and protocol implementations.

[mertalev]

Thanks for the PR @mertalev, but I think I should do this one myself. I've already got a WIP PR for this, and I'm trying to solve the problem not just for listener blocks, but also blocking blocks, and protocol implementations.

That's definitely fair! This PR is more limited in scope and you have much more context on the issue than I do. For now, this branch is useful to address the listener block crash for my app, so I'm fine with closing this PR and using it until your changes land. If it's useful, you can use as much or as little of this branch as you like.