feat: enhance API and session management with Nacos and Redis integration

- Add Nacos registry for service registration and deregistration.
- Implement Redis registry for session management with heartbeat and session claiming.
- Improve completion service with session handling and request validation.
- Enhance WebSocket handling for completion requests with JSON-RPC support.
- Add tests for new registry implementations and completion manager functionalities.
- Refactor existing code for better readability and maintainability.

backend/cmd/server/config_test.go

@@ -0,0 +1,90 @@
+package main
+
+import (
+	"os"
+	"path/filepath"
+	"strings"
+	"testing"
+	"time"
+
+	"monica-go-completion-backend/internal/completion"
+)
+
+func TestLoadConfigFileWithEnvOverride(t *testing.T) {
+	dir := t.TempDir()
+	path := filepath.Join(dir, "config.json")
+	content := `{
+		"port": "8081",
+		"requestTimeout": "3s",
+		"enableRedis": false,
+		"servers": [
+			{
+				"language": "go",
+				"languageId": "go",
+				"command": "gopls-from-file",
+				"args": []
+			}
+		]
+	}`
+	if err := os.WriteFile(path, []byte(content), 0o644); err != nil {
+		t.Fatalf("write config file failed: %v", err)
+	}
+
+	t.Setenv("CONFIG_FILE", path)
+	t.Setenv("PORT", "9090")
+	t.Setenv("GO_LSP_COMMAND", "gopls-from-env")
+
+	cfg, err := loadConfig()
+	if err != nil {
+		t.Fatalf("loadConfig() error = %v", err)
+	}
+
+	if cfg.Port != "9090" {
+		t.Fatalf("expected env to override port, got %s", cfg.Port)
+	}
+	if cfg.RequestTimeout != 3*time.Second {
+		t.Fatalf("expected requestTimeout=3s, got %s", cfg.RequestTimeout)
+	}
+	if cfg.EnableRedis {
+		t.Fatal("expected enableRedis=false from config file")
+	}
+
+	goSpec, ok := findServer(cfg.Servers, "go")
+	if !ok {
+		t.Fatal("expected go server in config")
+	}
+	if goSpec.Command != "gopls-from-env" {
+		t.Fatalf("expected go command overridden by env, got %s", goSpec.Command)
+	}
+}
+
+func TestLoadConfigFileInvalidDuration(t *testing.T) {
+	dir := t.TempDir()
+	path := filepath.Join(dir, "config.json")
+	content := `{
+		"requestTimeout": "not-a-duration"
+	}`
+	if err := os.WriteFile(path, []byte(content), 0o644); err != nil {
+		t.Fatalf("write config file failed: %v", err)
+	}
+
+	t.Setenv("CONFIG_FILE", path)
+
+	_, err := loadConfig()
+	if err == nil {
+		t.Fatal("expected loadConfig() to fail")
+	}
+	if !strings.Contains(err.Error(), "requestTimeout") {
+		t.Fatalf("expected requestTimeout parse error, got %v", err)
+	}
+}
+
+func findServer(specs []completion.LanguageServerSpec, language string) (completion.LanguageServerSpec, bool) {
+	target := strings.ToLower(strings.TrimSpace(language))
+	for _, spec := range specs {
+		if strings.ToLower(strings.TrimSpace(spec.Language)) == target {
+			return spec, true
+		}
+	}
+	return completion.LanguageServerSpec{}, false
+}