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/shared.go

@@ -15,7 +15,7 @@ import (
 	"context"
 	"encoding/json"
 	"fmt"
-	"log"
+	"log/slog"
 	"net/http"
 	"reflect"
 	"slices"
@@ -381,7 +381,8 @@ const (
 
 // notifySessions calls Notify on all the sessions.
 // Should be called on a copy of the peer sessions.
-func notifySessions[S Session, P Params](sessions []S, method string, params P) {
+// The logger must be non-nil.
+func notifySessions[S Session, P Params](sessions []S, method string, params P, logger *slog.Logger) {
 	if sessions == nil {
 		return
 	}
@@ -394,8 +395,7 @@ func notifySessions[S Session, P Params](sessions []S, method string, params P)
 	for _, s := range sessions {
 		req := newRequest(s, params)
 		if err := handleNotify(ctx, method, req); err != nil {
-			// TODO(#218): surface this error better
-			log.Printf("calling %s: %v", method, err)
+			logger.Warn(fmt.Sprintf("calling %s: %v", method, err))
 		}
 	}
 }