mcp/server: enable server logging (#501)

mcp/server.go: implement server‑side logging

- Thread a *slog.Logger through server paths to improve debuggability
- Introduce no‑op discard handler + ensureLogger so loggers are never
nil
- Use ensured loggers in Server
- address todo comments related to adding logging support

All tests are passing and no functionality should change with this PR

Fixes https://github.com/modelcontextprotocol/go-sdk/issues/170

mcp/logging.go

@@ -88,6 +88,23 @@ type LoggingHandler struct {
 	handler         slog.Handler
 }
 
+// discardHandler is a slog.Handler that drops all logs.
+// TODO: use slog.DiscardHandler when we require Go 1.24+.
+type discardHandler struct{}
+
+func (discardHandler) Enabled(context.Context, slog.Level) bool  { return false }
+func (discardHandler) Handle(context.Context, slog.Record) error { return nil }
+func (discardHandler) WithAttrs([]slog.Attr) slog.Handler        { return discardHandler{} }
+func (discardHandler) WithGroup(string) slog.Handler             { return discardHandler{} }
+
+// ensureLogger returns l if non-nil, otherwise a discard logger.
+func ensureLogger(l *slog.Logger) *slog.Logger {
+	if l != nil {
+		return l
+	}
+	return slog.New(discardHandler{})
+}
+
 // NewLoggingHandler creates a [LoggingHandler] that logs to the given [ServerSession] using a
 // [slog.JSONHandler].
 func NewLoggingHandler(ss *ServerSession, opts *LoggingHandlerOptions) *LoggingHandler {