[Question] Task listing pagination behavior clarification

For [list-tasks method](https://a2a-protocol.org/latest/specification/#314-list-tasks) the specification is explicit about ordering:

> Implementations MUST return tasks sorted by their last update time in descending order (most recently updated tasks first). This ensures consistent pagination and allows clients to efficiently monitor recent task activity.

The use of a non-monotonic mutable field as a pagination cursor makes it possible for tasks to be skipped. How do we know the last-updated ordering is going to be the most important / common / useful? Should the MUST requirement be relaxed to give implementers more flexibility?
```
// Tasks returned sorted by last_update DESC
|task1, task2|, task3

// If task3 changes before the next page is requested it is never returned:
task3, task1, task2 || <- last_update < task2.last_update
```

Would also be good to add a requirement level clarification for `pageSize` (must or should?).

### Code of Conduct

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Review the A2A specification section 3.14 list-tasks method to understand the current ordering requirement. Examine the protocol's pagination model and consider edge cases where tasks could be skipped due to updates. Look for existing implementations or discussions about task listing to gauge common usage patterns. Determine if the MUST should be relaxed to a SHOULD and clarify the pageSize requirement level.

Scritto dal modello di indicizzazione a partire dal testo della issue.