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 spring-projectsgh-33601

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

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