chore: improve sigstore-python maintenance path

[otjdiepluong]

Summary:

• Add focused unit tests for edge cases in the small helper functions in sigstore/_utils.py (e.g. base64 padding handling, PEM/DER conversion error paths, key id computation on empty/invalid inputs) to cover currently-untested error branches.
• Keep the change narrow so it is straightforward to review.

Notes:

• I kept this scoped to the relevant implementation and tests.