[objective_c] CI to check for xcode analyser errors

[liamappelbe]

The new CI step repro'd the error Stuart found, before I added the fixes: https://github.com/dart-lang/native/actions/runs/19284761993/job/55143210494

Fixes #2776

[coveralls]

coverage: 91.973% (+0.05%) from 91.924%
when pulling edacb2c on objc_warn
into 00991b9 on main.

[github-actions]

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// Copyright (c) 2025, the Dart project authors. Please see the AUTHORS file
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pkgs/objective_c/lib/src/ns_input_stream.dart

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Package	Leaked API symbol	Leaking sources
objective_c	_FinalizablePointer	internal.dart::_ObjCReference::new::_finalizable

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Breaking changes ✔️

Package	Change	Current Version	New Version	Needed Version	Looking good?
objective_c	None	9.0.0+1	9.1.0-dev	9.0.0+1	✔️

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