test: Improve test module return types and document external URL dependency

[thomhurst]

Summary

• Change test modules from Module<IDictionary<string, object>?> to Module<bool> where return value was always null (cleaner and more explicit)
• Add documentation comment to DownloaderTests explaining the external URL dependency

26 test files modified including:
AfterPipelineLoggerTests, AlwaysRunTests, AsyncDisposableModuleTests, DependsOnTests, DirectCollisionTests, DisposableModuleTests, EngineCancellationTokenTests, FailedPipelineTests, LoggingSecretTests, ModuleHistoryTests, ModuleLoggerTests, ModuleNotRegisteredExceptionTests, NestedCollisionTests, OneWayDependenciesNonCollisionTests, PipelineProgressTests, PipelineRequirementTests, ResultsRepositoryTests, RetryTests, RunnableCategoryTests, SafeEstimatedTimeProviderTests, SkipDependabotAttributeTests, TimedDependencyTests, UnusedModuleDetectorTests

Special cases:

• JsonSerializationTests: Uses non-nullable IDictionary<string, object> where actual dictionary values are needed
• FailedModuleWithCustomRetryPolicy: Uses string return type to work with IRetryable<T> interface

Fixes #1597, Fixes #1619

Test plan

• Build project to verify changes compile
• Verify test structure remains correct

🤖 Generated with Claude Code

[thomhurst]

Summary

Refactors test modules to use more appropriate return types: Module<bool> where tests always returned null, and Module<IDictionary<string, object>> (non-nullable) where tests return actual dictionary values.

Critical Issues

None found ✅

Suggestions

None - the changes are clean and improve code clarity. The non-nullable dictionary type in JsonSerializationTests correctly reflects that those modules always return actual dictionary instances, while the bool type for other test modules is more explicit than returning null from a nullable dictionary type.

Previous Review Status

No previous comments.

Verdict

✅ APPROVE - No critical issues

The changes improve test code readability and type safety without affecting functionality. All modified return types correctly reflect the actual behavior of the test modules.