mcp: re-enable race test after fixing data races (#851)

1. (non-test) Don't queue notifications if there are no connected
sessions.
2. Change `reconnectInitialDelay` to be atomic, as it is modified in
tests.
3. Protect map access with mutex in `TestServerTransportCleanup`.

mcp/streamable.go

@@ -26,6 +26,7 @@ import (
 	"strconv"
 	"strings"
 	"sync"
+	"sync/atomic"
 	"time"
 
 	"github.com/modelcontextprotocol/go-sdk/auth"
@@ -1515,9 +1516,13 @@ var (
 	// reconnectInitialDelay is the base delay for the first reconnect attempt.
 	//
 	// Mutable for testing.
-	reconnectInitialDelay = 1 * time.Second
+	reconnectInitialDelay atomic.Int64
 )
 
+func init() {
+	reconnectInitialDelay.Store(int64(1 * time.Second))
+}
+
 // Connect implements the [Transport] interface.
 //
 // The resulting [Connection] writes messages via POST requests to the
@@ -2196,7 +2201,7 @@ func calculateReconnectDelay(attempt int) time.Duration {
 		return 0
 	}
 	// Calculate the exponential backoff using the grow factor.
-	backoffDuration := time.Duration(float64(reconnectInitialDelay) * math.Pow(reconnectGrowFactor, float64(attempt-1)))
+	backoffDuration := time.Duration(float64(reconnectInitialDelay.Load()) * math.Pow(reconnectGrowFactor, float64(attempt-1)))
 	// Cap the backoffDuration at maxDelay.
 	backoffDuration = min(backoffDuration, reconnectMaxDelay)