Resolving warnings and treating warnings as errors in WinUI project.

[bkaankose]

Motivation

• Ensure the translator source generator reliably finds the en_US/resources.json file across platforms and path styles.
• Allow the source generator project to use the latest language features by setting LangVersion to latest.
• Surface nullable intent in tests to match nullable-enabled code and avoid warnings.
• Update CommunityToolkit.Mvvm to a newer build version used by the solution.

Description

• Normalize resource file path matching in TranslatorGenerator by replacing backslashes with forward slashes and using EndsWith("en_US/resources.json", StringComparison.OrdinalIgnoreCase).
• Register the post-init attribute and JSON schema tracking unchanged, and continue to only generate sources when the JSON schema changes.
• Add <LangVersion>latest</LangVersion> to Wino.SourceGenerators.csproj to enable up-to-date C# features for the generator.
• Change the test helper signature in ImapSynchronizerCalDavConfigurationTests.CreateSynchronizer to accept IAutoDiscoveryService? (nullable) for the optional parameter.
• Bump CommunityToolkit.Mvvm package version in Directory.Packages.props from 8.4.0 to 8.4.1-build.4.

Testing

• No automated tests were run as part of this change.

---

Codex Task

[github-actions]

Core test results

55 tests   45 ✅  3s ⏱️
 1 suites  10 💤
 1 files     0 ❌

Results for commit 127308c.

♻️ This comment has been updated with latest results.