Add Description property to Implementation type per MCP spec

[Copilot]

Implements the description field for the Implementation type as specified in modelcontextprotocol/modelcontextprotocol#1817.

Changes

• Protocol: Added optional Description property to Implementation class following the same pattern as Tool, Prompt, and Resource types
• Tests: Updated ImplementationTests to verify JSON serialization/deserialization of the Description field
• Integration: Updated McpClientTests to verify Description propagates correctly in both directions through the client-server handshake:
  • CanReadServerInfo test validates server's Description is accessible from the client
  • ServerCanReadClientInfo test validates client's Description is accessible from the server

Example

var serverInfo = new Implementation
{
    Name = "my-server",
    Version = "1.0.0",
    Description = "Provides access to customer database and analytics tools"
};

// Serializes to:
// {"name":"my-server","version":"1.0.0","description":"Provides access to..."}

The property is optional and nullable, maintaining backward compatibility with existing implementations.

> [!WARNING]
>
>

Original prompt

Add the Description property as spec'd in https://github.com/modelcontextprotocol/modelcontextprotocol/pull/1817/files to the Implementation client and server types. Update existing tests accordingly to ensure that such descriptions are propagated between client and server as expected.

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