Add file-based MCP server sample

[Meir017]

Description

Add a sample demonstrating the .NET 10 file-based programs feature for creating MCP servers. This would showcase how to build a complete MCP stdio server in a single .cs file without requiring a project (.csproj) file.

Motivation

File-based programs are ideal for:

• Learning the MCP SDK with minimal overhead
• Quick prototyping and experimentation
• Building simple command-line utilities
• Creating educational samples and tutorials

Currently, all samples in the repository use traditional project-based structures. A file-based sample would provide an alternative approach that's more accessible for newcomers and better suited for small, self-contained examples.

Proposed Implementation

Create samples/FileBasedMcpServer/ with a single Program.cs file that demonstrates:

Key Features

• Top-level statements - No explicit Main method boilerplate
• File-scoped types - Using the file keyword to keep types contained
• Unix shebang support - #!/usr/local/share/dotnet/dotnet run for direct execution
• Package directives - #:package for NuGet packages and #:project for local references
• No project file - Run directly with dotnet run Program.cs

Sample Content

• A complete MCP stdio server with hosting and DI
• Example tools (e.g., weather forecast tools)
• Proper logging configuration to stderr
• File-scoped classes and records

Documentation

• README explaining file-based programs
• Comparison with project-based samples
• Instructions for running on different platforms
• Guidance on when to use file-based vs project-based approaches

Reference

• File-based programs tutorial
• C# preprocessor directives for file-based apps

Benefits

• Lower barrier to entry for developers learning MCP
• Demonstrates modern C# 14 features
• Provides a lightweight alternative to full project structure
• Great for documentation and educational purposes