Create push notification config returns no id on database-backed stores

## What happens

Creating a push notification config against a database-backed server returns a config with no `id`. Reading it back with that id raises `InvalidParamsError`.

```
create returned id = ''
get FAILED -> InvalidParamsError: Validation failed
```

The same calls against the in-memory store return `id='task-123'` and the get succeeds.

## Why

`on_create_task_push_notification_config` returns the caller's request object rather than what the store persisted (`default_request_handler.py:556`, `default_request_handler_v2.py:394`).

Both stores default an empty `id` to the task id, on different objects:

- `inmemory_push_notification_config_store.py:51` sets it on the caller's object, so `return params` carries it.
- `database_push_notification_config_store.py:292` copies first and sets it on the copy, so `params` never gets it.

The store holds the right id either way; only the response differs.

## Version

main @ d55a3d3

The issue is in `default_request_handler.py` line 556 and `default_request_handler_v2.py` line 394, where `on_create_task_push_notification_config` returns the request `params` instead of the persisted object. Compare with `inmemory_push_notification_config_store.py:51` and `database_push_notification_config_store.py:292` to see how each store handles the `id`. Write a test that creates a push notification config using the database store and verifies the returned config has a valid `id`. Run the existing push notification tests to ensure the fix doesn't break anything.

索引モデルが issue の本文から書いたものです。