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_test.go

@@ -0,0 +1,61 @@
+package random
+
+import (
+	"errors"
+	"fmt"
+	"testing"
+)
+
+type brainrotReader struct {
+	err error
+}
+
+func (f *brainrotReader) Read(p []byte) (int, error) {
+	return 0, f.err
+}
+
+func TestRandom_String(t *testing.T) {
+	tests := []struct {
+		name    string
+		length  int
+		wantErr bool
+	}{
+		{"ValidLengthZero", 0, false},
+		{"ValidPositiveLength", 10, false},
+		{"NegativeLength", -1, true},
+		{"VeryLargeLength", 1_000_000, false},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			randomizer := New()
+
+			result, err := randomizer.String(tt.length)
+			if (err != nil) != tt.wantErr {
+				t.Errorf("String() error = %v, wantErr %v", err, tt.wantErr)
+				return
+			}
+
+			if !tt.wantErr && len(result) != tt.length {
+				t.Errorf("String() length = %v, want %v", len(result), tt.length)
+			}
+		})
+	}
+}
+
+func TestRandomWithFailingReader_String(t *testing.T) {
+	var randomizer Random
+	var errBrainrot = fmt.Errorf("you are not sigma enough")
+	randomizer = &random{reader: &brainrotReader{err: errBrainrot}}
+	t.Run("test failing reader", func(t *testing.T) {
+		result, err := randomizer.String(20)
+		if !errors.Is(err, errBrainrot) {
+			t.Errorf("String() error = %v, wantErr %v", err, errBrainrot)
+			return
+		}
+
+		if result != "" {
+			t.Errorf("String() result = %v, want an empty string due to error", result)
+		}
+	})
+}