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
• make resourceUri authoritative: require a concrete absolute ui:// URI, reject URI templates, and reject conflicting _meta.ui.resourceUri values
• preserve other existing UI metadata and fill in a missing UI resource URI
• reuse the existing resource delegate pipeline so HTML handlers can be synchronous or asynchronous and accept CancellationToken
• document and verify that duplicate resource URIs use the first registered HTML handler

Behavior

If _meta.ui.resourceUri is present, it must exactly match resourceUri; otherwise registration throws ArgumentException. When the UI object exists without a resource URI, the helper adds the authoritative value while preserving fields such as visibility. This guarantees that every registered tool link identifies the resource that can actually be read.

The helper accepts direct UI resource URIs only. Resource templates belong to the lower-level resource APIs and are rejected here because an app tool links to one concrete UI resource.

Testing

• dotnet test tests/ModelContextProtocol.Tests/ModelContextProtocol.Tests.csproj -f net10.0 --no-restore --filter FullyQualifiedName~WithAppTool (15 passed)
• dotnet test tests/ModelContextProtocol.Tests/ModelContextProtocol.Tests.csproj -f net10.0 --no-restore --filter FullyQualifiedName~McpApps (48 passed)
• dotnet test tests/ModelContextProtocol.Tests/ModelContextProtocol.Tests.csproj -f net10.0 --no-build --no-restore --filter (Execution!=Manual) (2371 passed)
• dotnet build src/ModelContextProtocol.Extensions.Apps/ModelContextProtocol.Extensions.Apps.csproj --configuration Release --no-restore --no-incremental (4 projects, 0 warnings, 0 errors)
• scoped dotnet format whitespace --verify-no-changes for the three changed C# files

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