fix(signing): decouple multisign's tx_list borrow from Transaction's lifetime

[soloking1412]

High Level Overview of Change

multisign's signature conflated two independent lifetimes under one parameter 'a: the borrow lifetime of tx_list and T's own Transaction<'a, F> data lifetime. Whenever a caller's T got inferred as 'static (routine when a transaction is built from string literals inside an async block), the borrow of tx_list was forced to 'static too, leaving Box::leak as the only way to call the function. Implements Option A from #306: give the borrow its own lifetime and take a slice instead of &Vec<T>.

// before
pub fn multisign<'a, T, F>(transaction: &mut T, tx_list: &'a Vec<T>) -> XRPLHelperResult<()>

// after
pub fn multisign<'a, 'b, T, F>(transaction: &mut T, tx_list: &'b [T]) -> XRPLHelperResult<()>

'b is the short borrow lifetime; 'a remains T's own data lifetime, now free to be inferred independently. The function body only iterates tx_list once and clones out each tx's first signer — it never stores the reference past the call, so a short, independent borrow is sufficient. &Vec<T> → &[T] is also more idiomatic and existing &vec call sites keep working via the standard slice coercion.

Context of Change

Filed as #306 after Box::leak showed up as the only way to satisfy the borrow checker in the multisign integration test added by #298.

Type of Change

• Bug fix (non-breaking change which fixes an issue)

Before / After

tests/transactions/multisign_payment.rs previously had to leak the signer-signed-copies Vec to get a &'static Vec<_>:

let signer_signed_copies: &'static Vec<_> =
    Box::leak(Box::new(vec![payment_signed_by_a, payment_signed_by_b]));
xrpl::transaction::multisign(&mut payment, signer_signed_copies)

Now it's a plain local borrow:

let signer_signed_copies = vec![payment_signed_by_a, payment_signed_by_b];
xrpl::transaction::multisign(&mut payment, &signer_signed_copies)

Test Plan

• cargo test --release: 1314 passed, including the existing unit test in src/transaction/multisign.rs::test.
• cargo check --tests --features std,json-rpc,helpers,cli,websocket,integration: compiles clean, confirming tests/transactions/multisign_payment.rs no longer needs Box::leak and matches CI's feature set (this test requires a live xrpld node to actually execute, so it wasn't run end-to-end here — only compiled).
• cargo clippy --all-targets --features std,json-rpc,helpers,cli,websocket,integration and cargo fmt --check: clean on both changed files.

Closes #306