[LOW] Reject incompatible verification key algorithms

[OskarEichler]

Summary

Reject verification keys from a different algorithm family during assignment. This keeps attacker-controlled key/algorithm mismatches inside the gem’s VerifyKeyError boundary instead of allowing later backend method failures.

RSA keys are validated by their public components, and EdDSA keys by their Ed25519 verification-key type. ECDSA already validates its algorithm family and curve.

Reproduction

The shared capability-only check accepts an ECDSA key for RSA because it responds to verify; RSA-PSS later raises NoMethodError for verify_pss. EdDSA similarly accepts RSA keys and fails later on its two-argument verification call. Credential formats that carry both algorithm identifiers and public keys can expose these mismatches to untrusted input.

Verification

• Focused model: three RSA-PSS/RSA-PKCS1/EdDSA mismatches fail on master and raise VerifyKeyError on this branch
• Existing suite: 31 examples, 0 failures on Ruby 4.0.6 through rbenv
• Normal wrapper and raw OpenSSL RSA verification keys remain accepted
• RuboCop, package build, and downstream loading passed in the cumulative ordinary review
• No repository tests or dependency files changed

Compatibility and breaking changes

Valid keys are unchanged. Incompatible keys now fail at assignment with VerifyKeyError; callers relying on a later backend-specific exception will observe this intentional error-timing change.