mcp: add StreamableHTTPOptions.SessionTimeout (#594)

Add a timeout option for the streamable HTTP handler that automatically
cleans up idle sessions.

Also, fix a bug in the streamable client, where we hang on a request
even though the client can never get a response (because the HTTP
request terminated without a response or Last-Event-Id).

Fixes #499

mcp/streamable_client_test.go

@@ -34,7 +34,6 @@ type streamableResponse struct {
 	body                string // or ""
 	optional            bool   // if set, request need not be sent
 	wantProtocolVersion string // if "", unchecked
-	callback            func() // if set, called after the request is handled
 }
 
 type fakeResponses map[streamableRequestKey]*streamableResponse
@@ -96,9 +95,6 @@ func (s *fakeStreamableServer) ServeHTTP(w http.ResponseWriter, req *http.Reques
 		http.Error(w, "no response", http.StatusInternalServerError)
 		return
 	}
-	if resp.callback != nil {
-		defer resp.callback()
-	}
 	for k, v := range resp.header {
 		w.Header().Set(k, v)
 	}
@@ -411,3 +407,38 @@ func TestStreamableClientStrictness(t *testing.T) {
 		})
 	}
 }
+
+func TestStreamableClientUnresumableRequest(t *testing.T) {
+	// This test verifies that the client fails fast when making a request that
+	// is unresumable, because it does not contain any events.
+	ctx := context.Background()
+	fake := &fakeStreamableServer{
+		t: t,
+		responses: fakeResponses{
+			{"POST", "", methodInitialize}: {
+				header: header{
+					"Content-Type":  "text/event-stream",
+					sessionIDHeader: "123",
+				},
+				body: "",
+			},
+			{"DELETE", "123", ""}: {optional: true},
+		},
+	}
+	httpServer := httptest.NewServer(fake)
+	defer httpServer.Close()
+
+	transport := &StreamableClientTransport{Endpoint: httpServer.URL}
+	client := NewClient(testImpl, nil)
+	cs, err := client.Connect(ctx, transport, nil)
+	if err == nil {
+		cs.Close()
+		t.Fatalf("Connect succeeded unexpectedly")
+	}
+	// This may be a bit of a change detector, but for now check that we're
+	// actually exercising the early failure codepath.
+	msg := "terminated without response"
+	if !strings.Contains(err.Error(), msg) {
+		t.Errorf("Connect: got error %v, want containing %q", err, msg)
+	}
+}