[objective_c] Various small fixes

[liamappelbe]

• Add URI versions of the mac SDK path variables. Fixes Consider Uri SDK variables #2763
• Update the documentation of ObjCObject and ObjCProtocol now that they're more publicly visible. Fixes ObjCObject documentation does not match usage #2759
• Export several FFIgen config API types. Fixes [Interop EAP] Missing exports used in FfiGenerator (ffigen v20.0.0) #2765

[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
// 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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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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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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[coveralls]

coverage: 82.536% (+0.6%) from 81.971%
when pulling 576c133 on objc_quick_fix
into 0e7789d on main.