mcp: export session missing error (#771)

Fix: #715

- export session missing error
- modify `errSessionMissing` to `ErrSessionMissing`
- add unit test about session missing error

mcp/streamable_test.go

@@ -2280,3 +2280,47 @@ data: {"jsonrpc":"2.0","method":"test2","params":{}}
 		}
 	}
 }
+
+func Test_ExportErrSessionMissing(t *testing.T) {
+	ctx := context.Background()
+
+	// 1. Setup server
+	impl := &Implementation{Name: "test", Version: "1.0.0"}
+	server := NewServer(impl, nil)
+	handler := NewStreamableHTTPHandler(func(r *http.Request) *Server { return server }, nil)
+	ts := httptest.NewServer(handler)
+	defer ts.Close()
+
+	// 2. Setup client
+	clientTransport := &StreamableClientTransport{
+		Endpoint: ts.URL,
+	}
+	client := NewClient(impl, nil)
+	session, err := client.Connect(ctx, clientTransport, nil)
+	if err != nil {
+		t.Fatalf("Connect failed: %v", err)
+	}
+	defer session.Close()
+
+	// 3. Manually invalidate session on server
+	handler.mu.Lock()
+	if len(handler.sessions) != 1 {
+		handler.mu.Unlock()
+		t.Fatalf("expected 1 session, got %d", len(handler.sessions))
+	}
+	for id := range handler.sessions {
+		delete(handler.sessions, id)
+	}
+	handler.mu.Unlock()
+
+	// 4. Try to call a tool (or any request)
+	_, err = session.ListTools(ctx, nil)
+	if err == nil {
+		t.Fatal("expected error, got nil")
+	}
+
+	// 5. Verify it's ErrSessionMissing
+	if !errors.Is(err, ErrSessionMissing) {
+		t.Errorf("expected error to wrap ErrSessionMissing, got: %v", err)
+	}
+}