feat: add SSE keepalive messages for connection health

Add configurable SSEKeepAlive interval (default: 15s) that sends SSE
comment lines (`:keepalive`) to maintain connection health.

Benefits:
- Keeps connections alive through proxies/load balancers that timeout
  idle connections
- Detects stale connections earlier (write failures terminate the
  handler)
- Standard SSE pattern - comments are ignored by compliant clients

Configuration:
- SSEKeepAlive > 0: send keepalives at specified interval
- SSEKeepAlive = 0: use default (15s)
- SSEKeepAlive < 0: disable keepalives

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

internal/mcp/transport_http_test.go

@@ -409,6 +409,69 @@ func TestHTTPTransportSSEStream(t *testing.T) {
 	}
 }
 
+func TestHTTPTransportSSEKeepalive(t *testing.T) {
+	transport, ts := testHTTPTransport(t, HTTPConfig{
+		SSEKeepAlive: 50 * time.Millisecond, // Short interval for testing
+	})
+
+	session, _ := transport.sessions.Create()
+
+	// Start SSE stream
+	req, _ := http.NewRequest("GET", ts.URL+"/mcp", nil)
+	req.Header.Set("Mcp-Session-Id", session.ID)
+	req.Header.Set("Accept", "text/event-stream")
+
+	resp, err := http.DefaultClient.Do(req)
+	if err != nil {
+		t.Fatalf("Request failed: %v", err)
+	}
+	defer resp.Body.Close()
+
+	if resp.StatusCode != http.StatusOK {
+		t.Fatalf("Expected 200, got %d", resp.StatusCode)
+	}
+
+	// Read with timeout - should receive keepalive within 100ms
+	buf := make([]byte, 256)
+	done := make(chan struct{})
+	var readData string
+	var readErr error
+
+	go func() {
+		n, err := resp.Body.Read(buf)
+		readData = string(buf[:n])
+		readErr = err
+		close(done)
+	}()
+
+	select {
+	case <-done:
+		if readErr != nil && readErr.Error() != "EOF" {
+			t.Fatalf("Read error: %v", readErr)
+		}
+		// Should receive SSE comment keepalive
+		if !strings.Contains(readData, ":keepalive") {
+			t.Errorf("Expected keepalive comment, got: %q", readData)
+		}
+	case <-time.After(200 * time.Millisecond):
+		t.Error("Timeout waiting for keepalive")
+	}
+}
+
+func TestHTTPTransportSSEKeepaliveDisabled(t *testing.T) {
+	server := NewServer(nil, log.New(io.Discard, "", 0))
+	config := HTTPConfig{
+		SSEKeepAlive: -1, // Explicitly disabled
+	}
+	transport := NewHTTPTransport(server, config)
+	defer transport.sessions.Stop()
+
+	// When SSEKeepAlive is negative, it should remain negative (disabled)
+	if transport.config.SSEKeepAlive != -1 {
+		t.Errorf("Expected SSEKeepAlive to remain -1 (disabled), got %v", transport.config.SSEKeepAlive)
+	}
+}
+
 func TestHTTPTransportParseError(t *testing.T) {
 	_, ts := testHTTPTransport(t, HTTPConfig{})
 
@@ -510,6 +573,9 @@ func TestHTTPTransportDefaultConfig(t *testing.T) {
 	if transport.config.ReadHeaderTimeout != DefaultReadHeaderTimeout {
 		t.Errorf("Expected default read header timeout %v, got %v", DefaultReadHeaderTimeout, transport.config.ReadHeaderTimeout)
 	}
+	if transport.config.SSEKeepAlive != DefaultSSEKeepAlive {
+		t.Errorf("Expected default SSE keepalive %v, got %v", DefaultSSEKeepAlive, transport.config.SSEKeepAlive)
+	}
 
 	transport.sessions.Stop()
 }