Fix concurrency related build issues

[SynergyMike]

This pull request updates the project to use Xcode 26 beta in CI workflows, adjusts platform deployment targets for macOS, and improves test reliability and actor-safety in the test suite. The most important changes are grouped below:

CI/CD Workflow Updates:

• Updated all CI jobs in .github/workflows/icy_sky.yml to use Xcode 26 beta instead of Xcode 16.1, ensuring builds and tests run against the latest Xcode version. [1] [2] [3]
• Added steps to download the iOS 26 Simulator in relevant CI jobs to match the updated Xcode version. [1] [2]

Platform Deployment Target Adjustments:

• Lowered the minimum macOS deployment target from .v26 to .v15 in both Packages/Features/Package.swift and Packages/Model/Package.swift, increasing compatibility. [1] [2]

Test Improvements and Actor Safety:

• Introduced an UpdatesCollector actor in AuthTests.swift to safely collect asynchronous updates, and refactored tests to use this actor for concurrency correctness. [1] [2] [3]
• Added @MainActor annotations to test structs and view builders in HeaderViewTests.swift, FeedsListTitleViewTests.swift, and AppTabsRoot.swift to ensure UI-related code runs on the main thread. [1] [2] [3]