mcp: fix a bug causing premature termination of streams

As reported in #227, there was a race where streams could be terminated
before the final reply was written. Fix this by checking the number of
outstanding requests in the same critical section that acquires outgoing
messages to write.

Fixes #227

mcp/streamable.go

@@ -267,9 +267,7 @@ type stream struct {
 
 	// streamRequests is the set of unanswered incoming RPCs for the stream.
 	//
-	// Lifecycle: requests values persist until the requests have been
-	// replied to by the server. Notably, NOT until they are sent to an HTTP
-	// response, as delivery is not guaranteed.
+	// Requests persist until their response data has been added to outgoing.
 	requests map[jsonrpc.ID]struct{}
 }
 
@@ -482,6 +480,7 @@ stream:
 		t.mu.Lock()
 		outgoing := stream.outgoing
 		stream.outgoing = nil
+		nOutstanding := len(stream.requests)
 		t.mu.Unlock()
 
 		for _, data := range outgoing {
@@ -493,9 +492,6 @@ stream:
 			}
 		}
 
-		t.mu.Lock()
-		nOutstanding := len(stream.requests)
-		t.mu.Unlock()
 		// If all requests have been handled and replied to, we should terminate this connection.
 		// "After the JSON-RPC response has been sent, the server SHOULD close the SSE stream."
 		// §6.4, https://modelcontextprotocol.io/specification/2025-06-18/basic/transports#sending-messages-to-the-server