Heartbeat scheduler/job limits are scoped per stream receiver instead of per component.

### Summary

Elastic Agent groups enabled inputs of one input type/output into a component in process mode:

https://github.com/elastic/elastic-agent/blob/main/pkg/component/component.go#L630-L686

OTel translation creates one complete Beat receiver per stream unless the input spec uses `single_receiver: true`:

https://github.com/elastic/elastic-agent/blob/main/internal/pkg/otel/translate/otelconfig.go#L470-L505

The Synthetics input specs do not set `single_receiver`. Every Heartbeat receiver calls `beater.New` and creates an independent scheduler:

https://github.com/elastic/beats/blob/main/heartbeat/beater/heartbeat.go#L111-L139

Heartbeat's default job limits are browser `2` and API `4`, and `SYNTHETICS_LIMIT_*` overrides are read into each scheduler configuration:

https://github.com/elastic/beats/blob/main/heartbeat/config/config.go#L64-L92

Consequently, the limits are no longer component/private-location-wide. With many browser/API monitor streams, every receiver owns a separate semaphore. This changes the effective concurrency bound from one limit per grouped Heartbeat component to one limit per monitor receiver.

### Expected

Heartbeat scheduler and `SYNTHETICS_LIMIT_*` limits continue to bound aggregate monitor concurrency for the component/private location.

### Actual

Each stream receiver independently enforces the configured limit.