fix: keep ledger_history in standalone mode for rwdb null node store

Since #728, [node_db] type=rwdb is the null node store. SHAMapStoreImp
sizes LedgerMaster's in-memory retained-ledger window from ledger_history
and throws when it is 0 or "full". Config::setup, however, unconditionally
forced LEDGER_HISTORY to 0 in standalone mode, so any standalone node
using rwdb (including the shipped cfg/rippled.cfg, and tools such as
xrpld-lab) aborted at startup with:

    RWDB null mode requires a finite ledger_history > 0

jtx builds its Config without Config::setup, so the unit test suite never
exercised this path.

Gate the standalone reset on !nullBackend() instead of removing it: for
persistent backends standalone behaviour is unchanged, and for rwdb the
configured value (default 256) is kept, matching what jtx already runs
with. cfg/rippled.cfg needs no change.

Add a Config_test case that goes through Config::setup in standalone mode
and checks ledger_history is kept for type=rwdb and still zeroed for a
persistent backend.

src/test/core/Config_test.cpp

@@ -1122,6 +1122,36 @@ trustthesevalidators.gov
         }
     }
 
+    // Standalone forces ledger_history to zero, but [node_db] type=rwdb
+    // (the null node store) sizes its retained-ledger window from it and
+    // SHAMapStoreImp throws on zero. cfg/rippled.cfg ships exactly this
+    // configuration, and jtx never calls Config::setup, so pin it here.
+    void
+    testSetupStandaloneRwdb()
+    {
+        testcase("setup standalone rwdb keeps ledger_history");
+
+        for (std::string const type : {"rwdb", "NuDB"})
+        {
+            detail::RippledCfgGuard const cfg(
+                *this,
+                "testSetupStandaloneRwdb_" + type,
+                "",
+                "",
+                true,
+                "[node_db]\ntype=" + type + "\npath=main\n");
+            Config config;
+            config.setup(
+                cfg.configFile(),
+                /*bQuiet*/ true,
+                /* bSilent */ true,
+                /* bStandalone */ true);
+            BEAST_EXPECT(config.standalone());
+            BEAST_EXPECT(config.nullBackend() == (type == "rwdb"));
+            BEAST_EXPECT(config.LEDGER_HISTORY == (type == "rwdb" ? 256 : 0));
+        }
+    }
+
     void
     testPort()
     {
@@ -1813,6 +1843,7 @@ r.ripple.com:51235
         testValidatorsFile();
         testSetup(false);
         testSetup(true);
+        testSetupStandaloneRwdb();
         testPort();
         testZeroPort();
         testWhitespace();

src/xrpld/core/detail/Config.cpp

@@ -405,7 +405,11 @@ Config::setup(
 
     HTTPClient::initializeSSLContext(*this, j_);
 
-    if (RUN_STANDALONE)
+    // Standalone never fetches history, so ledger_history is normally
+    // meaningless there. The rwdb null node store (nullBackend) uses it
+    // to size the in-memory retained-ledger window, and SHAMapStoreImp
+    // rejects zero for it, so keep the configured value in that case.
+    if (RUN_STANDALONE && !nullBackend())
         LEDGER_HISTORY = 0;
 
     std::string ledgerTxDbType;