fix: replace os.Setenv with t.Setenv in tests (usetesting)

Replace os.Setenv calls with t.Setenv in test functions to ensure
proper test environment cleanup and better test isolation.

internal/vault/vault_test.go

@@ -1,7 +1,6 @@
 package vault
 
 import (
-	"os"
 	"path/filepath"
 	"testing"
 
@@ -11,29 +10,12 @@ import (
 )
 
 func TestVaultOperations(t *testing.T) {
-	// Save original environment variables
-	oldMnemonic := os.Getenv(secret.EnvMnemonic)
-	oldPassphrase := os.Getenv(secret.EnvUnlockPassphrase)
-
-	// Clean up after test
-	defer func() {
-		if oldMnemonic != "" {
-			os.Setenv(secret.EnvMnemonic, oldMnemonic)
-		} else {
-			os.Unsetenv(secret.EnvMnemonic)
-		}
-
-		if oldPassphrase != "" {
-			os.Setenv(secret.EnvUnlockPassphrase, oldPassphrase)
-		} else {
-			os.Unsetenv(secret.EnvUnlockPassphrase)
-		}
-	}()
+	// Test environment will be cleaned up automatically by t.Setenv
 
 	// Set test environment variables
 	testMnemonic := "abandon abandon abandon abandon abandon abandon abandon abandon abandon abandon abandon about"
-	os.Setenv(secret.EnvMnemonic, testMnemonic)
-	os.Setenv(secret.EnvUnlockPassphrase, "test-passphrase")
+	t.Setenv(secret.EnvMnemonic, testMnemonic)
+	t.Setenv(secret.EnvUnlockPassphrase, "test-passphrase")
 
 	// Use in-memory filesystem
 	fs := afero.NewMemMapFs()