Set up solution scaffolding: Infrastructure & Tests projects, shared build settings

Goal

Establish the base Clean Architecture skeleton so that Domain, Application, Infrastructure and test projects build consistently.

Why

Currently the solution only has empty Domain and Application projects targeting net10.0. Before implementing any storage functionality we need a consistent project layout, shared build settings and code style so future PRs stay consistent.

Tasks

• Add src/AzStorage.Infrastructure/AzStorage.Infrastructure.csproj (referencing Domain & Application) for concrete implementations (Azure SDK, file system, etc.).
• Add tests/AzStorage.UnitTests and tests/AzStorage.IntegrationTests projects (xUnit) referencing the corresponding src projects.
• Add all new projects to AzStorage.slnx.
• Add a Directory.Build.props at repo root to centralize shared properties (TargetFramework, ImplicitUsings, Nullable, LangVersion, analyzers) instead of duplicating them per csproj.
• Add an .editorconfig with basic C# formatting/naming rules.

Acceptance Criteria

• dotnet build succeeds for the whole solution.
• dotnet test runs (even with zero tests) without errors.
• No project duplicates TargetFramework/Nullable/ImplicitUsings settings that are already defined in Directory.Build.props.

Start with AzStorage.slnx and the existing Domain and Application project files to understand the current layout and target settings. Add the named Infrastructure and xUnit test projects, root Directory.Build.props, and .editorconfig, then verify the acceptance criteria with dotnet build and dotnet test.