mcp: implement a concurrency model for calls

Implement the concurrency model described in #26: notifications are
synchronous, but calls are asynchronous (except for 'initialize'). To
achieve this, implement jsonrpc2.Async(ctx) to signal asynchronous
handling. This is simpler to use than returning ErrAsyncResponse and
calling Respond, and since this is an internal detail we don't need to
worry too much about whether it's idiomatic.

Add tests that verify both features, for both client and server.

Also:
- replace req.ID.IsValid with req.IsCall
- remove the methodHandler type as we can just use MethodHandler

Fixes #26

mcp/streamable_test.go

@@ -689,7 +689,7 @@ func TestStreamableServerTransport(t *testing.T) {
 					defer wg.Done()
 
 					for m := range out {
-						if req, ok := m.(*jsonrpc.Request); ok && req.ID.IsValid() {
+						if req, ok := m.(*jsonrpc.Request); ok && req.IsCall() {
 							// Encountered a server->client request. We should have a
 							// response queued. Otherwise, we may deadlock.
 							mu.Lock()