Fix unlocker rm to succeed when keychain item is missing

When removing a keychain unlocker, if the keychain item doesn't exist
(e.g., already manually deleted or vault synced from another machine),
the removal should still succeed since the goal is to remove the
unlocker and the keychain item being gone already satisfies that goal.

internal/secret/keychainunlocker_test.go

@@ -165,3 +165,20 @@ func TestKeychainLargeData(t *testing.T) {
 	// Clean up
 	_ = deleteFromKeychain(testItemName)
 }
+
+func TestDeleteNonExistentKeychainItem(t *testing.T) {
+	// Skip test if not on macOS
+	if runtime.GOOS != "darwin" {
+		t.Skip("Keychain tests only run on macOS")
+	}
+
+	// Ensure item doesn't exist
+	testItemName := "test-nonexistent-keychain-item-12345"
+	_ = deleteFromKeychain(testItemName)
+
+	// Deleting a non-existent item should NOT return an error
+	// This is important for cleaning up unlocker directories when the keychain item
+	// has already been removed (e.g., manually by user, or on a different machine)
+	err := deleteFromKeychain(testItemName)
+	assert.NoError(t, err, "Deleting non-existent keychain item should not return an error")
+}