mcp: fix stray log.Printf in notifySessions

I encountered these logs, and they were distracting and noisy. We
shouldn't log to stderr unless configured by the user. Thread through
the logger.

Also, use ensureLogger in the streamable client to simplify call sites.

Updates #708
Updates #684

mcp/streamable.go

@@ -1453,7 +1453,7 @@ func (t *StreamableClientTransport) Connect(ctx context.Context) (Connection, er
 		done:       make(chan struct{}),
 		maxRetries: maxRetries,
 		strict:     t.strict,
-		logger:     t.logger,
+		logger:     ensureLogger(t.logger), // must be non-nil for safe logging
 		ctx:        connCtx,
 		cancel:     cancel,
 		failed:     make(chan struct{}),
@@ -1555,9 +1555,7 @@ func (c *streamableClientConn) connectStandaloneSSE() {
 		// stream.
 		//
 		// Treat this like MethodNotAllowed in non-strict mode.
-		if c.logger != nil {
-			c.logger.Warn(fmt.Sprintf("got %d instead of 405 for standalone SSE stream", resp.StatusCode))
-		}
+		c.logger.Warn(fmt.Sprintf("got %d instead of 405 for standalone SSE stream", resp.StatusCode))
 		resp.Body.Close()
 		return
 	}
@@ -1686,9 +1684,7 @@ func (c *streamableClientConn) Write(ctx context.Context, msg jsonrpc.Message) e
 			// Some servers return 200, even with an empty json body.
 			//
 			// In strict mode, return an error to the caller.
-			if c.logger != nil {
-				c.logger.Warn(errMsg)
-			}
+			c.logger.Warn(errMsg)
 			if c.strict {
 				return errors.New(errMsg)
 			}