chore: Address inline review comments on Vault helpers

- Restore the doc comment on STAmount::isRounded that was lost when it
  moved out of LendingHelpers.cpp.
- Collapse LoanManage::defaultLoan's Vault-update block into a direct
  return of addVaultAssets, since there is no follow-up work after it.
- Mark removeVaultAssets's amount==0 early-return with LCOV_EXCL_LINE: it
  is a defensive short-circuit for a branch that is only reachable in a
  vanishingly rare edge case (final withdrawal from a vault whose
  sfAssetsAvailable has already been written down to zero) and is not
  reached by any current transactor-level test.

include/xrpl/protocol/STAmount.h

@@ -772,6 +772,10 @@ roundToAsset(
     return roundToScale(ret, scale);
 }
 
+/* Checks if a value is already rounded to the specified scale.
+ * Returns true if rounding down and rounding up produce the same result,
+ * indicating no further precision exists beyond the scale.
+ */
 [[nodiscard]] inline bool
 isRounded(Asset const& asset, Number const& value, std::int32_t scale)
 {

src/libxrpl/ledger/helpers/VaultHelpers.cpp

@@ -307,7 +307,7 @@ removeVaultAssets(
     applyRemoveVaultAssets(ctx.view, vault, amount, finalRemoval);
 
     if (amount == beast::kZero)
-        return tesSUCCESS;
+        return tesSUCCESS;  // LCOV_EXCL_LINE
 
     return doWithdraw(ctx, senderAcct, dstAcct, vault->at(sfAccount), priorBalance, amount, j);
 }

src/libxrpl/tx/transactors/lending/LoanManage.cpp

@@ -271,17 +271,13 @@ LoanManage::defaultLoan(
 
     // Update the Vault's assets, and return funds from the LoanBroker
     // pseudo-account to the Vault pseudo-account:
-    if (auto const ter = addVaultAssets(
-            view,
-            vaultSle,
-            brokerSle->at(sfAccount),
-            STAmount{vaultAsset, defaultCovered},
-            STAmount{vaultAsset, vaultTotalDelta},
-            j);
-        !isTesSuccess(ter))
-        return ter;
-
-    return tesSUCCESS;
+    return addVaultAssets(
+        view,
+        vaultSle,
+        brokerSle->at(sfAccount),
+        STAmount{vaultAsset, defaultCovered},
+        STAmount{vaultAsset, vaultTotalDelta},
+        j);
 }
 
 TER