chore(issuer): preserve contract errors in issuer schema registration

[alessandromazza98]

Very small chore PR that changes the IssuerError::Generic into a more specific SendTransactionError with transparent alloy error

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Note

Low Risk
Small error-type refactor limited to transaction send failure handling; behavior is effectively the same but downstream code may need to match the new IssuerError variant.

Overview
Updates issuer schema registration error handling to preserve the underlying Alloy contract error when .send() fails, replacing the previous string-wrapped IssuerError::Generic path with a typed IssuerError::SendTransactionError.

Removes the Generic error variant from IssuerError, making transaction-send failures more structured and inspectable by callers.

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