Make Result and subclasses final

[natebosch]

These were never intended for extension and marking them final makes
it easier to expand the API without breaking changes.

[github-actions]

PR Health

Unused Dependencies ✔️

Package	Status
async	✔️ All dependencies utilized correctly.

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Breaking changes ✔️

Package	Change	Current Version	New Version	Needed Version	Looking good?
async	Non-Breaking	2.13.1	2.14.0-wip	2.14.0-wip	✔️

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API leaks ✔️

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Coverage ⚠️

File	Coverage
pkgs/async/lib/async.dart	💔 Not covered
pkgs/async/lib/src/result/error.dart	💚 88 %
pkgs/async/lib/src/result/result.dart	💚 84 %
pkgs/async/lib/src/result/value.dart	💚 80 %

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License Headers ✔️

// Copyright (c) 2026, 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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no missing headers

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pkgs/collection/benchmark/legacy_quicksort.dart

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[natebosch]

This passed an internal test run, I think it's worth landing.