Trigger test webhook deliveries for a subscription

Send synthetic test events to verify webhook subscription configuration and endpoint availability. What this does: - Generates test events: Creates synthetic events for all event types in the subscription - Sends webhooks: Delivers test webhooks to the configured callback URL - Returns event IDs: Provides unique identifiers to track and verify test deliveries - Validates endpoint: Tests that your webhook endpoint can receive and process events

**Testing workflow:**
1. **Trigger**: Call this endpoint to send test events
2. **Track**: Use returned eventIds to monitor delivery status
3. **Verify**: Check your endpoint received the webhooks correctly
4. **Debug**: Use delivery records to troubleshoot issues

**Test event characteristics:**
- Contains sample data structure matching real events
- Includes metadata indicating it's a test event
- Follows same delivery flow as production webhooks
- Can be verified using the delivery verification endpoint

**Common use cases:**
- **Integration testing**: Verify your webhook handler works correctly
- **Endpoint validation**: Confirm callback URL is accessible and responds properly
- **Development**: Test changes without waiting for real events
- **Monitoring**: Verify subscription is active and functioning

**Important notes:**
- Test events are clearly marked as synthetic/test data
- Delivery records expire after 7 days (TTL)
- Use eventIds with GET /synthetic-delivery to verify delivery status
- Does not affect production event delivery or subscription configuration