[http_parser] Add named constructors to MediaType for convenience

[bambinoua]

• I’ve reviewed the contributor guide and applied the relevant portions to this PR.

Just few MediaType constructors added for convienience.

[github-actions]

PR Health

Breaking changes ⚠️

Package	Change	Current Version	New Version	Needed Version	Looking good?
http_parser	Non-Breaking	4.1.2	4.1.2	4.2.0 Got "4.1.2" expected >= "4.2.0" (non-breaking changes)	⚠️

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Changelog Entry ❗

Package	Changed Files
package:http_parser	pkgs/http_parser/lib/src/media_type.dart

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Coverage ⚠️

File	Coverage
pkgs/http_parser/lib/src/media_type.dart	💔 89 % ⬇️ 9 %

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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.

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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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Files
pkgs/http/example/main.dart
pkgs/http_client_conformance_tests/example/client_test.dart
pkgs/http_client_conformance_tests/lib/src/dummy_isolate.dart
pkgs/http_parser/test/example_test.dart
pkgs/http_profile/lib/http_profile.dart
pkgs/web_socket/example/web_socket_example.dart
pkgs/web_socket/lib/testing.dart
pkgs/web_socket_conformance_tests/example/client_test.dart