test(random): add unit tests for random behavior

- Added unit tests to cover random string generation and error handling.
- Introduced Random interface and random struct for better abstraction.
- Updated server, session, and interaction packages to require Random interface for dependency injection.

internal/random/random.go

@@ -1,12 +1,35 @@
 package random
 
-import "crypto/rand"
+import (
+	"crypto/rand"
+	"fmt"
+	"io"
+)
 
-func GenerateRandomString(length int) (string, error) {
+var (
+	ErrInvalidLength = fmt.Errorf("invalid length")
+)
+
+type Random interface {
+	String(length int) (string, error)
+}
+
+type random struct {
+	reader io.Reader
+}
+
+func New() Random {
+	return &random{reader: rand.Reader}
+}
+
+func (ran *random) String(length int) (string, error) {
+	if length < 0 {
+		return "", ErrInvalidLength
+	}
 	const charset = "abcdefghijklmnopqrstuvwxyz0123456789"
 	b := make([]byte, length)
 
-	if _, err := rand.Read(b); err != nil {
+	if _, err := ran.reader.Read(b); err != nil {
 		return "", err
 	}