feat(docker): add /healthz endpoint and -healthcheck flag (#254)

Adds a Docker healthcheck and the small HTTP/CLI plumbing it needs.

- Expose `GET /healthz` on the HTTP-mode server.
- Add `-healthcheck` to dial `http://127.0.0.1:<port>/healthz` and exit with status.
- Add a Dockerfile `HEALTHCHECK` instruction using `-healthcheck`.
- Document HTTP-mode usage and stdio override guidance.

Closes https://gitea.com/gitea/gitea-mcp/issues/146

Assisted by Codet.

Reviewed-on: https://gitea.com/gitea/gitea-mcp/pulls/254
Reviewed-by: silverwind <2021+silverwind@noreply.gitea.com>

operation/operation.go

@@ -133,6 +133,7 @@ func newHTTPServer(addr string, s *mcp.Server) *http.Server {
 			PropagateRequestCancellation: true,
 		},
 	)))
+	mux.HandleFunc("/healthz", handleHealthz)
 	return &http.Server{
 		Addr:              addr,
 		Handler:           mux,
@@ -140,6 +141,12 @@ func newHTTPServer(addr string, s *mcp.Server) *http.Server {
 	}
 }
 
+func handleHealthz(w http.ResponseWriter, _ *http.Request) {
+	w.Header().Set("Content-Type", "text/plain; charset=utf-8")
+	w.WriteHeader(http.StatusOK)
+	_, _ = w.Write([]byte("ok\n"))
+}
+
 func Run() error {
 	mcpServer = newMCPServer(flag.Version)
 	RegisterTool(mcpServer)

operation/operation_test.go

@@ -1,6 +1,10 @@
 package operation
 
-import "testing"
+import (
+	"net/http"
+	"net/http/httptest"
+	"testing"
+)
 
 func TestNewHTTPServerConfig(t *testing.T) {
 	server := newHTTPServer(":12345", newMCPServer("test"))
@@ -18,6 +22,21 @@ func TestNewHTTPServerConfig(t *testing.T) {
 	}
 }
 
+func TestHealthzEndpoint(t *testing.T) {
+	server := newHTTPServer(":0", newMCPServer("test"))
+
+	req := httptest.NewRequest(http.MethodGet, "/healthz", nil)
+	rec := httptest.NewRecorder()
+	server.Handler.ServeHTTP(rec, req)
+
+	if rec.Code != http.StatusOK {
+		t.Errorf("status = %d, want %d", rec.Code, http.StatusOK)
+	}
+	if body := rec.Body.String(); body == "" {
+		t.Error("body is empty, want a non-empty health message")
+	}
+}
+
 func TestParseAuthToken(t *testing.T) {
 	tests := []struct {
 		name      string