BED-8747: Check-in to BHE during collection

[StranDutton]

OpenHound stops making any API calls to BHE while a collection job runs. BHE kills the job after the 10-minute client check-in timeout.

This PR adds a periodic check-in so BHE knows the collector is still alive throughout a collection.

Changes:

• Hard-coded POLL_INTERVAL = 30 in service.py.
• Removed interval parameter from Service.init.
• Updated _poll() to branch based on job status:
• Idle: Calls check_jobs() to look for new jobs
• Active: Accesses self.client.jobs_current to perform a check-in
  Configuration & Cleanup:
• Removed all interval parsing and loading logic from src/scheduler.py.
• Purged DESTINATION__BLOODHOUNDENTERPRISE__INTERVAL and related interval keys from:
• deployments/helm/openhound/templates/deployment.yaml
• deployments/helm/openhound/values.yaml
• deployments/helm/values.example.yaml
• All .dlt-example/secrets_*.toml files.
  Tests:
• Unit tests added
• Updated test fixtures to remove the interval argument.

/resolves BED-8747

[StranDutton]

closing so i can base the work on feature branch