fix(dartpad-preview): cancel embedded workers during startup

[LeonMollNetlight]

When an embedded runtime is suspended during worker startup, its native worker currently keeps initializing until the session handshake returns a DartPad client. Rapid activation of several embeds can accumulate pending workers beyond the active runtime limit.

This change gives each startup attempt a Completer<void> and passes its future to DartPadSdk.dedicatedWorker(abortTrigger: ...). Suspension and close complete the trigger, allowing the SDK to terminate the pending worker before the handshake. Existing generation checks remain, and cleanup from a retired attempt cannot clear a resumed attempt’s trigger.

Draft dependency

This PR depends on the SDK client change on LeonMollNetlight/sdk:dartpad-cancel-worker-startup, which adds Future<void>? abortTrigger.

[github-actions]

PR Health

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.

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
no missing headers

All source files should start with a license header.

Unrelated files missing license headers

Files
pkgs/dart_services/lib/src/context.dart
pkgs/dartpad_preview/examples/counter/lib/main.dart
pkgs/dartpad_preview/examples/fibonacci/bin/main.dart
pkgs/dartpad_preview/examples/snippet_dart/bin/main.dart
pkgs/dartpad_preview/examples/sunflower/lib/main.dart
pkgs/samples/lib/brick_breaker.dart
pkgs/samples/lib/fibonacci.dart
pkgs/samples/lib/hello_world.dart
pkgs/samples/lib/main.dart
pkgs/samples/lib/sunflower.dart

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

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.

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.