[Fix](cloud-mow) Fix Fe memory leak due to txnLastSignatureMap (#59385)

### What problem does this PR solve?

introduced in #50417

entry in `txnLastSignatureMap` is not cleared when the commit is
launched by BE

Author: bobhan1

fe/fe-core/src/main/java/org/apache/doris/cloud/transaction/CloudGlobalTransactionMgr.java

@@ -428,6 +428,8 @@ public void commitTransactionWithoutLock(long dbId, List<Table> tableList, long
             }
             commitTransactionWithoutLock(dbId, tableList, transactionId, tabletCommitInfos, txnCommitAttachment, false,
                     mowTableList, backendToPartitionInfos);
+            // clear signature after commit succeeds
+            clearTxnLastSignature(dbId, transactionId);
         } catch (Exception e) {
             if (!mowTableList.isEmpty()) {
                 LOG.warn("commit txn {} failed, release delete bitmap lock, catch exception {}", transactionId,
@@ -1498,6 +1500,8 @@ public boolean commitAndPublishTransaction(DatabaseIf db, long transactionId,
             }
             commitTransactionWithSubTxns(db.getId(), tableList, transactionId, subTransactionStates, mowTableList,
                     backendToPartitionInfos);
+            // clear signature after commit succeeds
+            clearTxnLastSignature(db.getId(), transactionId);
         } catch (Exception e) {
             if (!mowTableList.isEmpty()) {
                 LOG.warn("commit txn {} failed, release delete bitmap lock, catch exception {}", transactionId,
@@ -1679,6 +1683,8 @@ public boolean commitAndPublishTransaction(DatabaseIf db, List<Table> tableList,
         beforeCommitTransaction(tableList, transactionId, timeoutMillis);
         try {
             commitTransactionWithoutLock(db.getId(), tableList, transactionId, tabletCommitInfos, txnCommitAttachment);
+            // Only clear signature after commit succeeds, as BE may retry on failure
+            clearTxnLastSignature(db.getId(), transactionId);
         } finally {
             stopWatch.stop();
             long costTimeMs = stopWatch.getTime();
@@ -1746,6 +1752,7 @@ public void abortTransaction(Long dbId, Long transactionId, String reason,
             abortTxnResponse = abortTransactionImpl(dbId, transactionId, reason, null);
         } finally {
             handleAfterAbort(abortTxnResponse, txnCommitAttachment, transactionId);
+            clearTxnLastSignature(dbId, transactionId);
         }
     }