Treat raw ECDSA signatures as binary strings

[OskarEichler]

Summary

Use bytesize and byteslice when detecting and splitting raw ECDSA signatures.

Reproduction

A valid 64-byte raw P-256 signature tagged with UTF-8 can have a character length below 64 when random bytes form valid multibyte sequences. The current character-based check skips raw-to-DER conversion and rejects the signature even though its bytes are unchanged.

Verification

• Isolated and cumulative branches: 31 examples, 0 failures on Ruby 4.0.6
• Focused valid raw-signature model fails on release/current when character length differs and passes
• Cumulative model, RuboCop (20 files), package build and downstream loading pass

DER signatures and binary-encoded raw signatures retain existing behavior.