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/version_test.go

@@ -255,10 +255,11 @@ func TestSecretVersionGetValue(t *testing.T) {
 	require.NoError(t, err)
 
 	// Retrieve the value
-	retrievedValue, err := sv.GetValue(ltIdentity)
+	retrievedBuffer, err := sv.GetValue(ltIdentity)
 	require.NoError(t, err)
+	defer retrievedBuffer.Destroy()
 
-	assert.Equal(t, expectedValue, retrievedValue)
+	assert.Equal(t, expectedValue, retrievedBuffer.Bytes())
 }
 
 func TestListVersions(t *testing.T) {