feat: add minimal flutter hello world sample (#3512)

[NamanGoyalK]

This PR adds a "Minimal Flutter" template to the samples dropdown, as discussed in #3512.

Changes:

Created pkgs/samples/lib/hello_world_flutter.dart based on the --empty template logic.

Updated samples.json to include the new sample in the Flutter category.

Renamed the existing Dart "Hello World" sample internal ID to hello-world-dart for clarity.

Re-generated pkgs/dartpad_ui/lib/primitives/samples.g.dart to sync changes.

This provides users with a clean-slate starting point for prototyping without the boilerplate of the counter app.

Closes #3512

---

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

[github-actions]

PR Health

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.

Files
pkgs/samples/lib/hello_world_dart.dart

All source files should start with a license header.

Unrelated files missing license headers

Files
pkgs/dart_services/lib/src/context.dart
pkgs/samples/lib/brick_breaker.dart
pkgs/samples/lib/fibonacci.dart
pkgs/samples/lib/main.dart
pkgs/samples/lib/sunflower.dart

This check can be disabled by tagging the PR with skip-license-check.

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

This check can be disabled by tagging the PR with skip-leaking-check.

Breaking changes ✔️

Package	Change	Current Version	New Version	Needed Version	Looking good?

This check can be disabled by tagging the PR with skip-breaking-check.

Unused Dependencies ✔️

Package	Status
None	No packages found to check.

For details on how to fix these, see dependency_validator.

This check can be disabled by tagging the PR with skip-unused-dependencies-check.

Coverage ✔️

File	Coverage

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

This check can be disabled by tagging the PR with skip-coverage-check.

Changelog Entry ✔️

Package	Changed Files

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

This check can be disabled by tagging the PR with skip-changelog-check.

[NamanGoyalK]

@parlough could you please review this once whenever you are free, I have fixed all the issues with ci/cd now, thanks!