Add WithAppTool helper for MCP Apps

[KirschBluteX]

Fixes #1607

Summary

• add WithAppTool to register a tool and its linked HTML app resource in one call
• preserve tool options and explicit _meta.ui, use text/html;profile=mcp-app, and enable the MCP Apps capability
• document the helper and cover registration, metadata, arguments, resource content, duplicate URIs, and invalid configuration

Testing

• dotnet test tests/ModelContextProtocol.Tests/ModelContextProtocol.Tests.csproj -f net10.0 --no-build --no-restore --filter (Execution!=Manual) (2362 passed)
• dotnet test tests/ModelContextProtocol.Tests/ModelContextProtocol.Tests.csproj -f net10.0 --no-build --no-restore --filter FullyQualifiedName~McpApps (39 passed)
• dotnet test tests/ModelContextProtocol.Tests/ModelContextProtocol.Tests.csproj -f net10.0 --no-build --no-restore --filter FullyQualifiedName~WithAppTool (6 passed after the final validation change)
• dotnet build src/ModelContextProtocol.Extensions.Apps/ModelContextProtocol.Extensions.Apps.csproj --no-restore --configuration Release --no-incremental (4 projects, 0 warnings, 0 errors)
• dotnet format whitespace ... --verify-no-changes --include ... for the three changed C# files

A full solution restore/build was also attempted with the offline package cache. Restore lacked runtime.win-x86.Microsoft.DotNet.ILCompiler 9.0.11 for TestServerWithHosting; the subsequent build failed only for the existing QuickstartWeatherServer and InMemoryTransport samples because runtime.win-x64.Microsoft.DotNet.ILCompiler 8.0.22 was unavailable. The Apps package and test projects built successfully.