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>

cmd/healthcheck_test.go

@@ -0,0 +1,81 @@
+package cmd
+
+import (
+	"bytes"
+	"net"
+	"net/http"
+	"net/http/httptest"
+	"strconv"
+	"strings"
+	"testing"
+)
+
+func TestRunHealthcheck(t *testing.T) {
+	tests := []struct {
+		name       string
+		handler    http.HandlerFunc
+		port       func(server *httptest.Server) int
+		wantOK     bool
+		wantStdout string
+	}{
+		{
+			name: "server responds 200 OK",
+			handler: func(w http.ResponseWriter, r *http.Request) {
+				w.WriteHeader(http.StatusOK)
+			},
+			port:       serverPort,
+			wantOK:     true,
+			wantStdout: "healthy",
+		},
+		{
+			name: "server responds with an error status",
+			handler: func(w http.ResponseWriter, r *http.Request) {
+				w.WriteHeader(http.StatusInternalServerError)
+			},
+			port:   serverPort,
+			wantOK: false,
+		},
+		{
+			name:   "nothing listening on the port",
+			port:   func(*httptest.Server) int { return 1 },
+			wantOK: false,
+		},
+	}
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			var server *httptest.Server
+			if tt.handler != nil {
+				server = httptest.NewServer(tt.handler)
+				defer server.Close()
+			}
+
+			var stdout, stderr bytes.Buffer
+			ok := runHealthcheck(http.DefaultClient, tt.port(server), &stdout, &stderr)
+
+			if ok != tt.wantOK {
+				t.Errorf("runHealthcheck() = %v, want %v", ok, tt.wantOK)
+			}
+			if tt.wantStdout != "" && !strings.Contains(stdout.String(), tt.wantStdout) {
+				t.Errorf("stdout = %q, want it to contain %q", stdout.String(), tt.wantStdout)
+			}
+			if tt.wantOK && stderr.Len() != 0 {
+				t.Errorf("stderr = %q, want empty on success", stderr.String())
+			}
+			if !tt.wantOK && stderr.Len() == 0 {
+				t.Error("stderr is empty, want a failure message")
+			}
+		})
+	}
+}
+
+func serverPort(server *httptest.Server) int {
+	_, portStr, err := net.SplitHostPort(server.Listener.Addr().String())
+	if err != nil {
+		panic(err)
+	}
+	port, err := strconv.Atoi(portStr)
+	if err != nil {
+		panic(err)
+	}
+	return port
+}