Adds new tests to the `pkg/crypt/openpgp` package to cover error handling and edge cases in the `EncryptPGP` and `DecryptPGP` functions.
- Adds a new test file, `encrypt_extra_test.go`, with tests for incorrect passphrases, malformed messages, and signing failures.
- Adds a pre-generated, encrypted PGP key to be used in tests, working around a limitation in the `go-crypto` library that prevents programmatic generation of encrypted keys.
Co-authored-by: google-labs-jules[bot] <161369871+google-labs-jules[bot]@users.noreply.github.com>
