Add MCP Apps UI support to Repl.Mcp

[carldebilly]

Summary

This PR adds MCP Apps support to Repl.Mcp, allowing a Repl command to open a host-rendered HTML UI from the same MCP server that already exposes tools, resources, and prompts.

This makes it possible for an MCP client that supports MCP Apps to show a small dashboard, viewer, form, or app surface directly from a Repl command, while clients without MCP Apps support still receive a normal text fallback instead of raw HTML.

What this enables

• A command can act as both the MCP Apps launcher and the UI resource.
• The UI can be generated by a normal Repl handler with .AsMcpAppResource().
• The UI handler can use dependency injection and the regular Repl pipeline.
• Generated UI resources can receive route parameters, including parameters from nested contexts.
• Tool calls return launcher text, while resource reads return the generated HTML.
• Hosts that support display modes can receive a display preference, while hosts that only support inline rendering still work.

The sample MCP server now includes a minimal contacts dashboard to demonstrate the intended pattern without introducing a large frontend app yet.

Documentation

The documentation has been updated to make MCP Apps a featured capability:

• root README
• Repl.Mcp README
• MCP server guide
• MCP advanced guide
• best practices
• comparison matrix
• architecture notes
• glossary
• sample documentation

The docs also call out that MCP Apps support is experimental in this version: UI handlers currently return generated HTML as string, Task<string>, or ValueTask<string>, and rendering/display modes remain host-dependent.

Validation

• dotnet test --project src\Repl.McpTests\Repl.McpTests.csproj
  • Passed: 128/128
• dotnet build samples\08-mcp-server\McpServerSample.csproj --output artifacts\verify\08-mcp-server
  • Passed
• git diff --check
  • Passed