[Bug]: ListTaskPushNotificationConfig paging is not coherent with ListTasks paging

### What happened?

In ListTasksRequest, the page size is defined as:
```
// Maximum number of tasks to return. Must be between 1 and 100.
// Defaults to 50 if not specified.
optional int32 page_size = 3;
```
while in ListTaskPushNotificationConfigRequest:

```
// The maximum number of configurations to return.
int32 page_size = 2;
```

I think both should be handled in the same way

### Relevant log output

```shell

```

### Code of Conduct

- [x] I agree to follow this project's Code of Conduct

Find the protobuf definitions for ListTasksRequest and ListTaskPushNotificationConfigRequest. Compare the page_size field handling (optional vs. required, default values). Ensure both are consistent, likely making the second optional with a default. Look for existing tests around pagination to verify the change.

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