Allow notification handlers post-creation temporary registration (#223)

- We still allow notification handlers to be configured pre-creation, eliminating any race conditions that may result from missed notifications. But now we also allow for post-creation notification handler registration.
- Handlers may now also be removed. This permits temporary registrations that may be created in a scoped manner.
- Registration handling is thread-safe.
- Handlers are now cancelable.
- Exceptions from notification handlers trigger all normal exception handling in the dispatch pipeline.

This also:
- Adds a WithStreamServerTransport as a counterpart to WithStdioServerTransport. The latter was used in several tests in a hacky way, and that's now simplified via the former's existence.

tests/ModelContextProtocol.Tests/Client/McpClientExtensionsTests.cs

@@ -26,9 +26,7 @@ public class McpClientExtensionsTests : LoggedTest
     {
         ServiceCollection sc = new();
         sc.AddSingleton(LoggerFactory);
-        sc.AddMcpServer().WithStdioServerTransport();
-        // Call WithStdioServerTransport to get the IMcpServer registration, then overwrite default transport with a pipe transport.
-        sc.AddSingleton<ITransport>(new StreamServerTransport(_clientToServerPipe.Reader.AsStream(), _serverToClientPipe.Writer.AsStream()));
+        sc.AddMcpServer().WithStreamServerTransport(_clientToServerPipe.Reader.AsStream(), _serverToClientPipe.Writer.AsStream());
         for (int f = 0; f < 10; f++)
         {
             string name = $"Method{f}";