filer: add ObjectTransactionBatch for multi-key object writes (#9649)

A multi-object delete spans many keys that route to different owner filers. The
gateway groups keys by owner and sends one batch per owner; the filer applies
each transaction under its own per-path lock, independent of the others.

A failed transaction (precondition or mutation error) is reported in its own
response without aborting the rest, matching S3 multi-object semantics where
each key succeeds or fails on its own. There is no cross-key atomicity, which S3
batch delete does not require.

other/java/client/src/main/proto/filer.proto

@@ -34,6 +34,9 @@ service SeaweedFiler {
     rpc ObjectTransaction (ObjectTransactionRequest) returns (ObjectTransactionResponse) {
     }
 
+    rpc ObjectTransactionBatch (ObjectTransactionBatchRequest) returns (ObjectTransactionBatchResponse) {
+    }
+
     rpc AtomicRenameEntry (AtomicRenameEntryRequest) returns (AtomicRenameEntryResponse) {
     }
     rpc StreamRenameEntry (StreamRenameEntryRequest) returns (stream StreamRenameEntryResponse) {
@@ -344,6 +347,19 @@ message ObjectTransactionResponse {
     FilerError error_code = 2;
 }
 
+// ObjectTransactionBatch applies several object transactions in one round trip,
+// each under its own per-path lock and independent of the others (no cross-key
+// atomicity). A caller groups keys that route to the same owner filer and sends
+// one batch per owner, e.g. for a multi-object delete. Each response is parallel
+// to its request.
+message ObjectTransactionBatchRequest {
+    repeated ObjectTransactionRequest transactions = 1;
+}
+
+message ObjectTransactionBatchResponse {
+    repeated ObjectTransactionResponse responses = 1;
+}
+
 message CreateEntryResponse {
     string error = 1;                          // kept for human readability + backward compat
     SubscribeMetadataResponse metadata_event = 2;