Expand automated coverage for update flows and platform-specific behavior

[HituziANDO]

Problem

The current tests cover date conversion, basic version comparisons, release-note storage, and environment selection, but not the main update-check flow, retries, built-in conditions, concurrency, presentation adapters, or macOS-specific behavior.

Scope

• Add state-transition tests for every UpdateCheckResult path.
• Use injected HTTP, clock, and state dependencies so tests never call external services.
• Add coverage for all built-in version-check and review-request conditions.
• Configure a macOS test target or scheme with meaningful testables.
• Replace assertions that cannot fail, such as checking a non-optional Int for nil.

Dependencies

• Record daily version checks only after a successful lookup #14
• Extract an injectable HTTP client and typed App Store response decoder #20
• Extract UpdateCheckService and add a structured async API #23

Acceptance criteria

• Success, failure, retry, empty response, update, latest, skip, and new-release paths are tested.
• Tests are independent of execution order, wall-clock time, and the public App Store service.
• macOS-specific logic runs in CI-capable tests.
• Concurrent-call regressions have deterministic tests.

Tests

• This issue is complete when the new suites pass repeatedly with randomized test order where supported.

Relevant files

• Framework/SwiftyUpdateKitTests/
• Framework/SwiftyUpdateKit.xcodeproj/xcshareddata/xcschemes/