Update the GET /rest.php/v1/campaignevents/participant-events API to support a new ongoing=true parameter that returns only ongoing events for the participant default is false.
**The API should:**
- Accept the new ongoing=true query parameter
- Filter events to return only those that are currently ongoing (within start/end date range)
- Return the same event structure but filtered by ongoing status
- Maintain backward compatibility when the parameter is not provided
- Always read from cache instead of querying the database directly
BDD
```
Feature: Filter participant ongoing events
Scenario: Get all participant events (backward compatibility)
Given a user has registered for multiple events
When the API is called without the ongoing parameter
Then all events for the participant are returned
And the response includes both ongoing and non-ongoing events
Scenario: Get only ongoing participant events
Given a user has registered for multiple events
And some events are currently ongoing
When the API is called with ongoing=true parameter
Then only ongoing events are returned
And non-ongoing events are filtered out
And the response structure remains the same
Scenario: No ongoing events for participant
Given a user has registered for events
And none of the events are currently ongoing
When the API is called with ongoing=true parameter
Then an empty list is returned
And the response indicates no ongoing events
Scenario: API reads from cache -> TBD see https://phabricator.wikimedia.org/T401211 for more details
Given the cache contains participant events data
When the API is called with ongoing=true parameter
Then the API reads from cache instead of database
And no database queries are performed
```