mcp: refactor requested changes to StreamID and comments

- Removed the StreamID type wrapper around empty strings
- fixed the incorrect comment citing defaultStreamID's

Author: 2bitburrito

mcp/streamable.go

@@ -182,7 +182,7 @@ func NewStreamableServerTransport(sessionID string, opts *StreamableServerTransp
 		streams:        make(map[StreamID]*stream),
 		requestStreams: make(map[jsonrpc.ID]StreamID),
 	}
-	t.streams[StreamID("")] = newStream(StreamID(""))
+	t.streams[""] = newStream("")
 	if opts != nil {
 		t.opts = *opts
 	}
@@ -240,7 +240,7 @@ type StreamableServerTransport struct {
 // at any time.
 type stream struct {
 	// id is the logical ID for the stream, unique within a session.
-	// defaultStreamID is used for messages that don't correlate with an incoming request.
+	// an empty string is used for messages that don't correlate with an incoming request.
 	id StreamID
 
 	// signal is a 1-buffered channel, owned by an incoming HTTP request, that signals