Problem

signing_pub_key in CommonFields is typed as Option<Cow<'a, str>>, but the XRPL protocol always expects this field to be present — either as a hex public key (signed transaction) or an empty string (multi-signed or unsigned). The current new() constructor already papers over this by wrapping the value: Some(signing_pub_key.unwrap_or("".into())), but consumers using the default/builder pattern can still set it to None, which is never valid on-ledger.

Proposed change

• Change CommonFields.signing_pub_key from Option<Cow<'a, str>> to Cow<'a, str> with #[serde(default)] so deserialization still works when the field is absent (defaults to "")
• Remove PreparedTransaction and SignedTransaction wrapper types — they exist only to carry signing_pub_key and txn_signature, both of which are already on CommonFields
• Update all transaction model constructors to pass "".into() instead of None for signing_pub_key
• Update is_signed() checks from .signing_pub_key.is_some() to !self.signing_pub_key.is_empty()
• Update the TransactionBuilder::with_signing_pub_key setter accordingly

Context

Originally proposed in #107 by @LimpidCrypto but that PR is now stale with 22 merge conflicts against main due to subsequent transaction model refactors. This issue captures the same intent for a fresh implementation.