Fix deferred service dependency health check

[adamf-oai]

Summary

• wait for the actual non-deferred dependency when starting a deferred service
• stop handling the start request after a dependency health-check failure
• cover the behavior with a deferred service that depends on a healthy non-deferred service

Root cause

handleStart iterated over dependency labels but called WaitUntilHealthy on the deferred service being started. Because that service has no process yet, the health check can dereference a nil process and break the /start request.

Validation

• bazel test //deferred:deferred_service_test --test_output=errors --nocache_test_results from tests/
• bazel test //... --test_output=errors from tests/ (43 tests passed)