fix(rpc): standardize error codes in doTransactionEntry (RIPD-7750)

Replace 4 ad-hoc jvResult[jss::error] string assignments with
rpc::injectError() calls using registered error codes. Responses
now include numeric error_code and error_message fields consistent
with all other RPC handlers.

Mapping:
  "fieldNotFoundTransaction" → RpcInvalidParams (missing tx_hash)
  "notYetImplemented"        → RpcNotImpl       (current ledger)
  "malformedRequest"         → RpcInvalidParams (bad hex)
  "transactionNotFound"      → RpcTxnNotFound   (tx absent)

Add <xrpl/protocol/ErrorCodes.h> include. Remove stale XXX comments.
Update TransactionEntry_test to assert error_code numerically.

Closes: https://github.com/XRPLF/rippled/issues/6798
RIPD-7750

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/test/rpc/TransactionEntry_test.cpp

@@ -36,9 +36,10 @@ class TransactionEntry_test : public beast::unit_test::Suite
                 })};
 
         {
-            // no params
+            // no params — AC-1: missing tx_hash → invalidParams
             auto const result = env.client().invoke("transaction_entry", {})[jss::result];
-            BEAST_EXPECT(result[jss::error] == "fieldNotFoundTransaction");
+            BEAST_EXPECT(result[jss::error] == "invalidParams");
+            BEAST_EXPECT(result[jss::error_code] == RpcInvalidParams);
             BEAST_EXPECT(result[jss::status] == "error");
         }
 
@@ -51,21 +52,25 @@ class TransactionEntry_test : public beast::unit_test::Suite
         }
 
         {
+            // AC-2: current ledger (no closed ledger) → notImpl
             json::Value params{json::ValueType::Object};
             params[jss::ledger] = "current";
             params[jss::tx_hash] = "DEADBEEF";
             auto const result = env.client().invoke("transaction_entry", params)[jss::result];
-            BEAST_EXPECT(result[jss::error] == "notYetImplemented");
+            BEAST_EXPECT(result[jss::error] == "notImpl");
+            BEAST_EXPECT(result[jss::error_code] == RpcNotImpl);
             BEAST_EXPECT(result[jss::status] == "error");
         }
 
         {
+            // AC-3: tx_hash is not valid hex → invalidParams
             json::Value params{json::ValueType::Object};
             params[jss::ledger] = "closed";
             params[jss::tx_hash] = "DEADBEEF";
             auto const result = env.client().invoke("transaction_entry", params)[jss::result];
             BEAST_EXPECT(!result[jss::ledger_hash].asString().empty());
-            BEAST_EXPECT(result[jss::error] == "malformedRequest");
+            BEAST_EXPECT(result[jss::error] == "invalidParams");
+            BEAST_EXPECT(result[jss::error_code] == RpcInvalidParams);
             BEAST_EXPECT(result[jss::status] == "error");
         }
 
@@ -126,8 +131,10 @@ class TransactionEntry_test : public beast::unit_test::Suite
         {
             // Valid structure, but transaction not found.
             json::Value const result{env.rpc("transaction_entry", txHash, "closed")};
+            // AC-4: valid hex but tx not in ledger → txnNotFound
             BEAST_EXPECT(!result[jss::result][jss::ledger_hash].asString().empty());
-            BEAST_EXPECT(result[jss::result][jss::error] == "transactionNotFound");
+            BEAST_EXPECT(result[jss::result][jss::error] == "txnNotFound");
+            BEAST_EXPECT(result[jss::result][jss::error_code] == RpcTxnNotFound);
             BEAST_EXPECT(result[jss::result][jss::status] == "error");
         }
     }

src/xrpld/rpc/handlers/transaction/TransactionEntry.cpp

@@ -7,6 +7,7 @@
 #include <xrpl/basics/chrono.h>
 #include <xrpl/json/json_value.h>
 #include <xrpl/ledger/ReadView.h>
+#include <xrpl/protocol/ErrorCodes.h>
 #include <xrpl/protocol/jss.h>
 
 #include <memory>
@@ -18,7 +19,7 @@ namespace xrpl {
 //   ledger_index : <ledger_index>
 // }
 //
-// XXX In this case, not specify either ledger does not mean ledger current. It
+// Note: not specifying either ledger does not mean ledger current — it
 // means any ledger.
 json::Value
 doTransactionEntry(rpc::JsonContext& context)
@@ -31,30 +32,30 @@ doTransactionEntry(rpc::JsonContext& context)
 
     if (!context.params.isMember(jss::tx_hash))
     {
-        jvResult[jss::error] = "fieldNotFoundTransaction";
+        // AC-1: missing tx_hash parameter
+        rpc::injectError(RpcInvalidParams, jvResult);
     }
     else if (jvResult.get(jss::ledger_hash, json::ValueType::Null).isNull())
     {
         // We don't work on ledger current.
-
-        // XXX We don't support any transaction yet.
-        jvResult[jss::error] = "notYetImplemented";
+        // AC-2: no closed ledger specified
+        rpc::injectError(RpcNotImpl, jvResult);
     }
     else
     {
         uint256 uTransID;
-        // XXX Relying on trusted WSS client. Would be better to have a strict
-        // routine, returning success or failure.
         if (!uTransID.parseHex(context.params[jss::tx_hash].asString()))
         {
-            jvResult[jss::error] = "malformedRequest";
+            // AC-3: tx_hash is not valid hex
+            rpc::injectError(RpcInvalidParams, jvResult);
             return jvResult;
         }
 
         auto [sttx, stobj] = lpLedger->txRead(uTransID);
         if (!sttx)
         {
-            jvResult[jss::error] = "transactionNotFound";
+            // AC-4: transaction not found in ledger
+            rpc::injectError(RpcTxnNotFound, jvResult);
         }
         else
         {