[appengine] Make ClientContext context getter nullable

[martin-dehlan]

What

Changes the top-level context getter in package:appengine to return
ClientContext? instead of ClientContext.

Why

The getter cast the service-scope lookup with as ClientContext. When called
outside of a request handler, the lookup returns null, so the cast threw a
runtime error instead of yielding null. Returning a nullable type lets callers
check for the absence of a request context safely.

Fixes #175

Notes

• This is a breaking API change (non-nullable → nullable return type), so the
  package version is bumped 0.13.12 → 0.14.0 with a CHANGELOG entry.
• dart analyze passes; no in-repo call sites or tests use the public getter.

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[sigurdm]

I'm not really sure we want to do this. I think one should really know at the call-site if they are inside a request handler or not. This makes the common case more troublesome because now you need a ! for each legitimate use site, not really providing any benefit.

[github-actions]

Package publishing

If you have publishing permissions, you can use the links below to publish the changes after merging this PR.

Package	Version	Status	Publish tag (post-merge)
package:appengine	0.14.0	ready to publish	appengine-v0.14.0
package:gcloud	0.9.0	already published at pub.dev
package:native_synchronization	0.3.1	already published at pub.dev
package:timezone	0.11.0	already published at pub.dev

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

[github-actions]

PR Health

Unused Dependencies ✔️

Package	Status
appengine	✔️ All dependencies utilized correctly.

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Breaking changes ✔️

Package	Change	Current Version	New Version	Needed Version	Looking good?
appengine	Breaking	0.13.12	0.14.0	0.14.0	✔️

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Coverage ✔️

File	Coverage
pkgs/appengine/lib/appengine.dart	💚 13 %

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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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Files
pkgs/timezone/example/example.dart
pkgs/timezone/test/datetime_browser_test.dart
pkgs/timezone/test/datetime_test.dart
pkgs/timezone/test/datetime_test_no_database.dart
pkgs/timezone/test/two_way_test.dart
pkgs/timezone/test/zicfile_test.dart
pkgs/timezone/test/zone_tab_test.dart
pkgs/timezone/tool/encode_dart.dart
pkgs/timezone/tool/encode_tzf.dart
pkgs/timezone/tool/get.dart
pkgs/timezone/tool/test_random.dart
pkgs/timezone/tool/zone_dump.dart

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