Signing key hex assumption can crash with non-hex keys

[jmarbutt]

Problem

The out-of-band registration code assumes the signing key suffix is hex-encoded and calls Convert.FromHexString(). Any non-hex signing key will throw a FormatException before registration completes.

Location: Inngest/InngestClient.cs:942-955

var keyBytes = Convert.FromHexString(normalizedKey);  // Throws if not valid hex

Expected Behavior

The SDK should either:

1. Handle non-hex keys gracefully (encode as UTF-8 bytes instead)
2. Validate the key format and return a clear error (401/500) instead of throwing

SDK Spec Reference

The spec only requires hashing the key bytes - it doesn't mandate hex encoding. We should verify what format Inngest actually sends keys in.

Priority

High - this could crash production applications during registration.