fix: Reject messages to tasks in terminal states (#760)

Per A2A spec CORE-SEND-002, SendMessage must return
UnsupportedOperationError when attempting to send messages to tasks in
terminal states (completed, canceled, rejected, failed).

Added validation in DefaultRequestHandler.initMessageSend() to check if
the existing task is in a terminal state (using TaskState.isFinal()) and
throw UnsupportedOperationError before the message reaches the
AgentExecutor.

This fixes the issue on all three transports (JSON-RPC, gRPC, HTTP+JSON)
since they all use the same DefaultRequestHandler code path.

Fixes #741 🦕

Co-authored-by: Claude Sonnet 4.5 <noreply@anthropic.com>

Author: kabir

server-common/src/main/java/io/a2a/server/requesthandlers/DefaultRequestHandler.java

@@ -1045,6 +1045,13 @@ private MessageSendSetup initMessageSend(MessageSendParams params, ServerCallCon
                         task.id(), task.contextId(), messageContextId));
             }
 
+            // Per spec CORE-SEND-002: Reject messages to tasks in terminal states
+            if (task.status().state().isFinal()) {
+                throw new UnsupportedOperationError(null, String.format(
+                        "Cannot send message to task %s: task is in terminal state %s and cannot accept further messages",
+                        task.id(), task.status().state()), null);
+            }
+
             LOGGER.debug("Found task updating with message {}", params.message());
             task = taskManager.updateWithMessage(params.message(), task);