fix(rest): map invalid history length to InvalidParamsError (#715)

## Description

Fixes REST `getTask` error handling when `historyLength` is invalid.

Previously, invalid `historyLength` values could throw
`IllegalArgumentException`
and be returned as an internal error (500). This change maps that case
to
`InvalidParamsError`, returning the correct client error semantics
(422).

## Changes

- Update `RestHandler#getTask(...)` to catch `IllegalArgumentException`
and return `InvalidParamsError`
- Add regression test:
  - `RestHandlerTest#testGetTaskNegativeHistoryLengthReturns422`

## Validation

- Ran targeted REST handler tests and verified they pass

Fixes #714

---------

Co-authored-by: Emmanuel Hugonnet <ehsavoie@users.noreply.github.com>

Author: Zhang-Charlie

transport/rest/src/main/java/io/a2a/transport/rest/handler/RestHandler.java

@@ -415,7 +415,12 @@ public HTTPRestResponse subscribeToTask(ServerCallContext context, String tenant
      */
     public HTTPRestResponse getTask(ServerCallContext context, String tenant, String taskId, @Nullable Integer historyLength) {
         try {
-            TaskQueryParams params = new TaskQueryParams(taskId, historyLength, tenant);
+            TaskQueryParams params;
+            try {
+                params = new TaskQueryParams(taskId, historyLength, tenant);
+            } catch (IllegalArgumentException e) {
+                throw new InvalidParamsError(e.getMessage());
+            }
             Task task = requestHandler.onGetTask(params, context);
             if (task != null) {
                 return createSuccessResponse(200, io.a2a.grpc.Task.newBuilder(ProtoUtils.ToProto.task(task)));

transport/rest/src/test/java/io/a2a/transport/rest/handler/RestHandlerTest.java

@@ -58,6 +58,17 @@ public void testGetTaskNotFound() {
         Assertions.assertTrue(response.getBody().contains("TaskNotFoundError"));
     }
 
+    @Test
+    public void testGetTaskNegativeHistoryLengthReturns422() {
+        RestHandler handler = new RestHandler(CARD, requestHandler, internalExecutor);
+
+        RestHandler.HTTPRestResponse response = handler.getTask(callContext, "", MINIMAL_TASK.id(), -1);
+
+        Assertions.assertEquals(422, response.getStatusCode());
+        Assertions.assertEquals("application/problem+json", response.getContentType());
+        Assertions.assertTrue(response.getBody().contains("InvalidParamsError"));
+    }
+
     @Test
     public void testListTasksStatusWireString() {
         RestHandler handler = new RestHandler(CARD, requestHandler, internalExecutor);