Support using WithStreamServerTransport for multiple in-process mcp servers

The current implementation register the stream support as a singleton:

builder.Services.AddSingleton<ITransport>(new StreamServerTransport(inputStream, outputStream));

This prevents declaring multiple MCP Server classes in-process and using them with a separate channels.

Reasons for this choice:

• creating MCP servers in a separate class library so that they can be instantiated in-process or out-of-process depending on the app
• ability to clearly separate the tools, prompts and resources. Currently I have to mix all the in-process prompts, resources and tools in a single MCP server to be used in-process.
• avoid creating a separate process just because of the library limitation (performance reasons)

As I mentioned in a discussion, the Model Context Protocol adhere to the "transparency" principle, but the library makes it very difficult to reach. This is the reason why I tagged this as a bug.

Start at the builder.Services.AddSingleton(new StreamServerTransport(inputStream, outputStream)) registration and inspect how StreamServerTransport is wired to an in-process MCP Server. Trace the channel and service-lifetime assumptions, then verify that multiple MCP Server classes can be declared in-process with separate channels.