fix(xrp): Don't set LastLedgerSequence when not provided in SigningInput

[MJamalian]

Summary

• Guard LastLedgerSequence assignment in both build_tx_json and prepare_builder to only set the field when the proto input value is non-zero
• When not provided (proto default 0), the field stays None and is omitted from serialization, matching the old C++ behavior

Problem

After the C++ → Rust migration in #4250, XRP transactions that don't explicitly set lastLedgerSequence in SigningInput get LastLedgerSequence = 0 in the signed output. This means "expire at ledger 0", causing immediate transaction expiry on the XRP Ledger.

The root cause is in protobuf_builder.rs:

// build_tx_json path:
if self.input.last_ledger_sequence != 0 || tx.common_fields.last_ledger_sequence.is_none() {
    tx.common_fields.last_ledger_sequence = Some(self.input.last_ledger_sequence);
}

// prepare_builder path:
builder.last_ledger_sequence(self.input.last_ledger_sequence)

When lastLedgerSequence is not provided, the proto default is 0. The condition false || true = true sets Some(0), and prepare_builder unconditionally sets it. Since CommonFields.last_ledger_sequence is Option<u32> with skip_serializing_if = "Option::is_none", Some(0) gets serialized into the transaction.

Fix

Only set the field when the input value is non-zero:

// build_tx_json path:
if self.input.last_ledger_sequence != 0 {
    tx.common_fields.last_ledger_sequence = Some(self.input.last_ledger_sequence);
}

// prepare_builder path:
if self.input.last_ledger_sequence != 0 {
    builder.last_ledger_sequence(self.input.last_ledger_sequence);
}

Note: the same pattern (!= 0 guard) is already correctly used for source_tag on line 339.

Test plan

• All existing Ripple tests pass unchanged (they all use non-zero last_ledger_sequence values)
• Verified with real XRP mainnet transactions that the fix produces valid output

Fixes #4731