Reuse NoTransactionInContextException instances

New NoTransactionInContextException instances are created frequently in
TransactionContextManager#currentContext() when there is no transaction,
we could reuse the same instance instead to reduce the GC pressure.

Closes gh-33601

spring-tx/src/main/java/org/springframework/transaction/reactive/TransactionContextManager.java

@@ -37,6 +37,9 @@ import org.springframework.transaction.NoTransactionException;
  */
 public abstract class TransactionContextManager {
 
+	private static final NoTransactionInContextException NO_TRANSACTION_IN_CONTEXT_EXCEPTION =
+			new NoTransactionInContextException();
+
 	private TransactionContextManager() {
 	}
 
@@ -60,7 +63,7 @@ public abstract class TransactionContextManager {
 					return Mono.just(holder.currentContext());
 				}
 			}
-			return Mono.error(new NoTransactionInContextException());
+			return Mono.error(NO_TRANSACTION_IN_CONTEXT_EXCEPTION);
 		});
 	}