Add support for XLS-85 Token Escrow (#5185)

- Specification: https://github.com/XRPLF/XRPL-Standards/pull/272
- Amendment: `TokenEscrow`
- Enables escrowing of IOU and MPT tokens in addition to native XRP.
- Allows accounts to lock issued tokens (IOU/MPT) in escrow objects, with support for freeze, authorization, and transfer rates.
- Adds new ledger fields (`sfLockedAmount`, `sfIssuerNode`, etc.) to track locked balances for IOU and MPT escrows.
- Updates EscrowCreate, EscrowFinish, and EscrowCancel transaction logic to support IOU and MPT assets, including proper handling of trustlines and MPT authorization, transfer rates, and locked balances.
- Enforces invariant checks for escrowed IOU/MPT amounts.
- Extends GatewayBalances RPC to report locked (escrowed) balances.

src/test/jtx/impl/Env.cpp

@@ -199,6 +199,48 @@ Env::balance(Account const& account, Issue const& issue) const
     return {amount, lookup(issue.account).name()};
 }
 
+PrettyAmount
+Env::balance(Account const& account, MPTIssue const& mptIssue) const
+{
+    MPTID const id = mptIssue.getMptID();
+    if (!id)
+        return {STAmount(mptIssue, 0), account.name()};
+
+    AccountID const issuer = mptIssue.getIssuer();
+    if (account.id() == issuer)
+    {
+        // Issuer balance
+        auto const sle = le(keylet::mptIssuance(id));
+        if (!sle)
+            return {STAmount(mptIssue, 0), account.name()};
+
+        STAmount const amount{mptIssue, sle->getFieldU64(sfOutstandingAmount)};
+        return {amount, lookup(issuer).name()};
+    }
+    else
+    {
+        // Holder balance
+        auto const sle = le(keylet::mptoken(id, account));
+        if (!sle)
+            return {STAmount(mptIssue, 0), account.name()};
+
+        STAmount const amount{mptIssue, sle->getFieldU64(sfMPTAmount)};
+        return {amount, lookup(issuer).name()};
+    }
+}
+
+PrettyAmount
+Env::limit(Account const& account, Issue const& issue) const
+{
+    auto const sle = le(keylet::line(account.id(), issue));
+    if (!sle)
+        return {STAmount(issue, 0), account.name()};
+    auto const aHigh = account.id() > issue.account;
+    if (sle && sle->isFieldPresent(aHigh ? sfLowLimit : sfHighLimit))
+        return {(*sle)[aHigh ? sfLowLimit : sfHighLimit], account.name()};
+    return {STAmount(issue, 0), account.name()};
+}
+
 std::uint32_t
 Env::ownerCount(Account const& account) const
 {