Remove sorting of allowedHelp maps

[natebosch]

Closes #845

It is idiomatic to treat the key order of a Dart map as meaningful
given that map literals and default Map type preserve key insertion
order. It is more useful to allow the caller to decide this order than
to mandate an alpha sorting by key. Callers which need this order can
construct the map appropriately, and callers which prefer a different
order now have the capability.

Remove the additional list copy and iterate the map keys directly.

Releasing as a non-breaking change since specific usage output is
considered an implementation detail. This is expected to impact some CI
statuf for packages with tests hardcoding a strict dependency on the
output.

No additional tests are necessary since updating the order in existing
tests demonstrates the same behavior as adding a non-sorting specific
test.

[github-actions]

Package publishing

Package	Version	Status	Publish tag (post-merge)
package:os_detect	2.0.3	ready to publish	os_detect-v2.0.3
package:args	2.6.1-wip	WIP (no publish necessary)
package:async	2.12.0	already published at pub.dev
package:characters	1.4.0	already published at pub.dev
package:collection	1.20.0-wip	WIP (no publish necessary)
package:convert	3.1.2	already published at pub.dev
package:crypto	3.0.6	already published at pub.dev
package:fixnum	1.1.1	already published at pub.dev
package:lints	5.1.1	already published at pub.dev
package:logging	1.3.0	already published at pub.dev
package:path	1.9.2-wip	WIP (no publish necessary)
package:platform	3.1.6	already published at pub.dev
package:typed_data	1.4.0	already published at pub.dev

Documentation at https://github.com/dart-lang/ecosystem/wiki/Publishing-automation.

[github-actions]

PR Health

Breaking changes ⚠️

Package	Change	Current Version	New Version	Needed Version	Looking good?
args	Non-Breaking	2.6.0	2.6.1-wip	2.7.0 Got "2.6.1-wip" expected >= "2.7.0" (non-breaking changes)	⚠️

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Changelog Entry ✔️

Package	Changed Files

Changes to files need to be accounted for in their respective changelogs.

Coverage ⚠️

File	Coverage
pkgs/args/lib/src/arg_parser.dart	💚 94 %
pkgs/args/lib/src/usage.dart	💔 99 % ⬇️ 0 %

This check for test coverage is informational (issues shown here will not fail the PR).

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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 symbols

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.

Files
no missing headers

All source files should start with a license header.

[jakemac53]

nit: consider an if case to extract this as a variable and avoid the !

[natebosch]

Done, did a couple nearby ones as well.