Fix GetValue methods to return LockedBuffer internally

- Changed Secret.GetValue and Version.GetValue to return *memguard.LockedBuffer
- Updated all internal callers to handle LockedBuffer properly
- For backward compatibility, vault.GetSecret still returns []byte but makes a copy
- This ensures secret values are protected in memory during decryption
- Updated tests to handle LockedBuffer returns
- Fixed CLI getSecretValue to use LockedBuffer throughout

internal/secret/secret.go

@@ -63,7 +63,7 @@ func NewSecret(vault VaultInterface, name string) *Secret {
 }
 
 // GetValue retrieves and decrypts the current version's value using the provided unlocker
-func (s *Secret) GetValue(unlocker Unlocker) ([]byte, error) {
+func (s *Secret) GetValue(unlocker Unlocker) (*memguard.LockedBuffer, error) {
 	DebugWith("Getting secret value",
 		slog.String("secret_name", s.Name),
 		slog.String("vault_name", s.vault.GetName()),