feat(server): add RpcServer for STDIO pipe server mode

[CalvinAllen]

Parent Epic

Part of #18 - STDIO Transport Support

Description

Create a new RpcServer class in the Server project that acts as a named pipe SERVER (inverse of existing RpcClient which is a pipe client).

Implementation

New File: src/CodingWithCalvin.MCPServer.Server/RpcServer.cs

• Mirror of existing RpcClient pattern but as NamedPipeServerStream
• WaitForConnectionAsync with configurable timeout (default 60s)
• Implements IServerRpc (for VS to call)
• Creates proxy to IVisualStudioRpc (to call VS)
• Handles connection/disconnection lifecycle

Key Methods

• StartAsync(string pipeName) - Create pipe server and start waiting
• WaitForConnectionAsync(TimeSpan timeout) - Block until VS connects or timeout
• Proxy implementations for all IVisualStudioRpc methods

Reference

Use src/CodingWithCalvin.MCPServer.Server/RpcClient.cs as pattern reference (invert server/client relationship).

Acceptance Criteria

• RpcServer class created
• Named pipe server functionality working
• Connection timeout handling
• Proper cleanup on dispose