.NET 10 preparation

[martincostello]

Cherry-pick changes from #3283:

• Remove usage of IWebHost in tests.
• Style tweaks and use of newer C# features.
• Update NuGet packages to their latest versions.
• Assert OpenAPI documents have no warnings.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 94.28%. Comparing base (c7cad21) to head (17bb5fb).
⚠️ Report is 1 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #3565   +/-   ##
=======================================
  Coverage   94.28%   94.28%           
=======================================
  Files         110      110           
  Lines        3816     3816           
  Branches      723      723           
=======================================
  Hits         3598     3598           
  Misses        218      218           

Flag	Coverage Δ
Linux	94.28% <ø> (ø)
Windows	94.28% <ø> (ø)
macOS	94.28% <ø> (ø)

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[Copilot]

Pull Request Overview

This PR prepares the codebase for .NET 10 by modernizing test infrastructure and updating dependencies. The changes focus on removing deprecated IWebHost usage in favor of the newer hosting model and applying various style improvements.

• Migrated test infrastructure from IWebHost to the generic IHost pattern
• Updated NuGet packages to their latest patch versions
• Added OpenAPI warnings validation to ensure document quality
• Applied C# style improvements including target-typed new expressions and trailing commas

Reviewed Changes

Copilot reviewed 19 out of 21 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
test/WebSites/WebApi/WebApi.csproj	Updated Microsoft.AspNetCore.OpenApi to version 9.0.9
test/WebSites/WebApi/EndPoints/OpenApiEndpoints.cs	Added trailing newline for consistent formatting
test/WebSites/ReDoc/Startup.cs	Added root redirect endpoint to api-docs
test/WebSites/OAuth2Integration/OAuth2Integration.csproj	Updated authentication packages to latest versions
test/WebSites/MvcWithNullable/MvcWithNullable.csproj	Updated Microsoft.AspNetCore.OpenApi to version 9.0.9
test/Swashbuckle.AspNetCore.IntegrationTests/VerifyTests.cs	Added outputHelper parameter to TestSiteAutofaq constructor
test/Swashbuckle.AspNetCore.IntegrationTests/TestSiteAutofaq.cs	Refactored to inherit from TestSite and use IHost instead of IWebHost
test/Swashbuckle.AspNetCore.IntegrationTests/TestSite.cs	Major refactor from IWebHost to IHost with improved configuration
test/Swashbuckle.AspNetCore.IntegrationTests/Swashbuckle.AspNetCore.IntegrationTests.csproj	Updated testing packages to latest versions
test/Swashbuckle.AspNetCore.IntegrationTests/SwaggerIntegrationTests.cs	Added warnings validation and TestSiteAutofaq constructor fix
test/Swashbuckle.AspNetCore.IntegrationTests/DocumentProviderTests.cs	Updated service access from Host.Services to Services
test/Swashbuckle.AspNetCore.IntegrationTests/CustomDocumentSerializerTests.cs	Updated service access from Host.Services to Services
test/Swashbuckle.AspNetCore.ApiTesting.Test/*.cs	Applied style improvements with trailing commas and target-typed new
test/Swashbuckle.AspNetCore.Annotations.Test/*.cs	Applied style improvements and simplified method declarations
src/Swashbuckle.AspNetCore.Cli/Program.cs	Added pragma warnings to suppress IWebHost deprecation warnings
Directory.Packages.props	Updated ASP.NET Core packages to latest patch versions

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