MINOR: Use TransactionOperation enum instead of String for pending state checks (#21218)

### PR Description
this PR refactors `TransactionManager#throwIfPendingState` to use a
typed `TransactionOperation` enum instead of a raw String.

###Background / Motivation
The previous String-based API relies on manually passing operation
names, which can make it easier for naming to drift or become
inconsistent over time. Using an enum makes the intent more explicit and
keeps the operation naming centralized.

### What’s changed
- Added a `TransactionOperation` enum with a `displayName` (and
`toString()` override) to preserve readable exception messages.
- Updated relevant call sites (`beginTransaction`, `prepareTransaction`,
`sendOffsetsToTransaction`, `maybeAddPartition`) to pass the
corresponding enum value.
- Updated `throwIfPendingState` to accept `TransactionOperation` instead
of `String`.

### Behavior / Compatibility
- No behavioral change is intended. The exception message remains
human-readable and consistent via `TransactionOperation#toString()`.

### Testing
No additional tests were added since this is a small refactor; existing
tests should continue to cover the pending-transition behavior.

Reviewers: TaiJuWu <tjwu1217@gmail.com>, Chia-Ping Tsai
 <chia7712@gmail.com>

Author: chickenchickenlove

clients/src/main/java/org/apache/kafka/clients/producer/internals/TransactionManager.java

@@ -205,6 +205,24 @@ private enum Priority {
             this.priority = priority;
         }
     }
+    
+    private enum TransactionOperation {
+        SEND("send"),
+        BEGIN_TRANSACTION("beginTransaction"),
+        PREPARE_TRANSACTION("prepareTransaction"),
+        SEND_OFFSETS_TO_TRANSACTION("sendOffsetsToTransaction");
+        
+        final String displayName;
+
+        TransactionOperation(String displayName) {
+            this.displayName = displayName;
+        }
+
+        @Override
+        public String toString() {
+            return displayName;
+        }
+    }
 
     public TransactionManager(final LogContext logContext,
                               final String transactionalId,
@@ -331,7 +349,7 @@ synchronized TransactionalRequestResult initializeTransactions(
 
     public synchronized void beginTransaction() {
         ensureTransactional();
-        throwIfPendingState("beginTransaction");
+        throwIfPendingState(TransactionOperation.BEGIN_TRANSACTION);
         maybeFailWithError();
         transitionTo(State.IN_TRANSACTION);
     }
@@ -343,7 +361,7 @@ public synchronized void beginTransaction() {
      */
     public synchronized void prepareTransaction() {
         ensureTransactional();
-        throwIfPendingState("prepareTransaction");
+        throwIfPendingState(TransactionOperation.PREPARE_TRANSACTION);
         maybeFailWithError();
         transitionTo(State.PREPARED_TRANSACTION);
         this.preparedTxnState = new ProducerIdAndEpoch(
@@ -406,7 +424,7 @@ private TransactionalRequestResult beginCompletingTransaction(TransactionResult
     public synchronized TransactionalRequestResult sendOffsetsToTransaction(final Map<TopicPartition, OffsetAndMetadata> offsets,
                                                                             final ConsumerGroupMetadata groupMetadata) {
         ensureTransactional();
-        throwIfPendingState("sendOffsetsToTransaction");
+        throwIfPendingState(TransactionOperation.SEND_OFFSETS_TO_TRANSACTION);
         maybeFailWithError();
 
         if (currentState != State.IN_TRANSACTION) {
@@ -438,7 +456,7 @@ public synchronized TransactionalRequestResult sendOffsetsToTransaction(final Ma
 
     public synchronized void maybeAddPartition(TopicPartition topicPartition) {
         maybeFailWithError();
-        throwIfPendingState("send");
+        throwIfPendingState(TransactionOperation.SEND);
 
         if (isTransactional()) {
             if (!hasProducerId()) {
@@ -1248,7 +1266,7 @@ private TxnOffsetCommitHandler txnOffsetCommitHandler(TransactionalRequestResult
         return new TxnOffsetCommitHandler(result, builder);
     }
 
-    private void throwIfPendingState(String operation) {
+    private void throwIfPendingState(TransactionOperation operation) {
         if (pendingTransition != null) {
             if (pendingTransition.result.isAcked()) {
                 pendingTransition = null;