[ffigen] Fix some edge cases in the error out param logic

[liamappelbe]

When updating the ObjC example, I noticed that the initWithContentsOfURL should be throwing the error, not taking it as an out-param. Turns out that this API was bypassing the heuristic because the param's originalName was outError, not error, and it's a nullable error pointer pointer. So I've updated the heuristic to handle these cases.

[github-actions]

PR Health

Breaking changes ✔️

Package	Change	Current Version	New Version	Needed Version	Looking good?
objective_c	Breaking	8.1.0	9.0.0-dev.0	9.0.0	✔️

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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
objective_c	_FinalizablePointer	internal.dart::_ObjCReference::new::_finalizable

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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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pkgs/objective_c/lib/src/ns_input_stream.dart

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[coveralls]

coverage: 82.509% (+0.06%) from 82.451%
when pulling c804da0 on ffigen_error_edge_cases
into d2edd93 on main.