mcp: fix panics with nil logger; ensure test handlers do not panic (#557)

PR 501 added logging to the streamable server, but didn't pass a non-nil
logger to the streamable transport (an oversight). However, tests still
passed due to net/http's panic recovery.

Update test servers to enforce that handlers do not panic, and fix the
panic by:
1. Passing along the configured logger to the session transport.
2. Using ensureLogger in Connect, since we technically allow
StreamableServerTransports to be constructed by the user.

Fixes #556

mcp/streamable.go

@@ -236,6 +236,7 @@ func (h *StreamableHTTPHandler) ServeHTTP(w http.ResponseWriter, req *http.Reque
 			SessionID:    sessionID,
 			Stateless:    h.opts.Stateless,
 			jsonResponse: h.opts.JSONResponse,
+			logger:       h.opts.Logger,
 		}
 
 		// To support stateless mode, we initialize the session with a default
@@ -377,6 +378,10 @@ type StreamableServerTransport struct {
 	// StreamableHTTPOptions.JSONResponse is exported.
 	jsonResponse bool
 
+	// optional logger provided through the [StreamableHTTPOptions.Logger].
+	//
+	// TODO(rfindley): logger should be exported, since we want to allow people
+	// to write their own streamable HTTP handler.
 	logger *slog.Logger
 
 	// connection is non-nil if and only if the transport has been connected.
@@ -393,7 +398,7 @@ func (t *StreamableServerTransport) Connect(ctx context.Context) (Connection, er
 		stateless:      t.Stateless,
 		eventStore:     t.EventStore,
 		jsonResponse:   t.jsonResponse,
-		logger:         t.logger,
+		logger:         ensureLogger(t.logger), // see #556: must be non-nil
 		incoming:       make(chan jsonrpc.Message, 10),
 		done:           make(chan struct{}),
 		streams:        make(map[string]*stream),