googleapis_auth: test cleanup

[kevmoo]

Modernized Assertions:

• Refactored numerous tests to use isA().having() for more precise and readable error checks.
• Replaced deprecated or verbose matcher patterns (e.g., expect(a, equals(b)) -> expect(a, b)).
• Standardized top-level matchers in test_utils.dart as const TypeMatcher.

Refined Test Structure:

• Removed single, redundant top-level group calls from across the test suite (adc_test.dart, iam_signer_test.dart, json_test.dart, pem_test.dart, etc.).
• Applied PascalCase to groups representing classes (e.g., group('AccessToken', ...)).
• Improved test descriptions to clearly state expected behavior (e.g., 'throws on invalid server response').

Cleaner Code:

• Moved to public API imports where appropriate.
• Standardized the placement of named parameters across all modernized tests.

All 107 tests passed after these changes. I'm ready for any further instructions!

[github-actions]

PR Health

Breaking changes ✔️

Package	Change	Current Version	New Version	Needed Version	Looking good?
googleapis_auth	Breaking	2.0.0	2.1.0-wip	2.1.0-wip	✔️

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Unused Dependencies ✔️

Package	Status
googleapis_auth	✔️ All dependencies utilized correctly.

For details on how to fix these, see dependency_validator.

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Changelog Entry ✔️

Package	Changed Files

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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.

Files
no missing headers

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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 symbol	Leaking sources

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