[codevoyager] fix: ChainerBackend falls through backends on PasswordDeleteError

[codevoyager-ai-dev]

The method previously only caught NotImplementedError, causing PasswordDeleteError from a higher-priority backend to propagate immediately without trying lower-priority backends. This meant that if a password was stored in a lower-priority backend (e.g. SecretService), but a higher-priority backend (e.g. DBusKeyring) was now available but did not have the password, the delete would fail with PasswordDeleteError: Password not found.

Fix: Catch PasswordDeleteError in addition to NotImplementedError in delete_password, so the chainer continues to the next backend when one raises PasswordDeleteError.

Testing:

• test_delete_password_falls_through_on_password_delete_error — verifies fall-through when highest-priority backend raises PasswordDeleteError
• test_delete_password_skips_not_implemented — verifies fall-through on NotImplementedError (existing behavior)
• All existing tests pass

Closes #697