[riha-dev]

FulcioVerifier.verifySigningCertificate accepts any end-entity certificate under a trusted-root CA as a signing certificate, because nothing checks the leaf's key usage or EKU (Java's PKIX validator only checks key usage on CA certificates):

• A leaf with only a serverAuth EKU, or with keyEncipherment as its only key usage, passes today, and KeylessVerifier.verify relies on this call for its certificate checks.
• The Fulcio certificate profile requires digitalSignature and codeSigning on every leaf. sigstore-python rejects certificates missing either, and sigstore-go passes ExtKeyUsageCodeSigning to x509.Verify.
• Public-good Fulcio certificates already carry both, so this mainly matters for private deployments whose trusted root lists a CA that also issues other kinds of certificates.
• The new FulcioVerifierTest cases issue both leaves from a test CA and fail on main. A code signing leaf from the same CA, the existing Fulcio fixtures and all 61 leaf certificates in the sigstore-conformance v0.0.29 assets still verify.