fix(healthcheck): use documented retry defaults

[fpytloun]

Summary

This is a small follow-up split out from review feedback on #25662.

It changes HealthConfig::default() to match the same retry defaults already used when deserializing HealthConfig from config:

• retry_initial_backoff_secs: 1
• retry_max_duration_secs: 3600s

Without this, code paths that construct a missing health config with HealthConfig::default() get zero retry timing values instead of the documented/deserialized defaults.

Change Type

• Bug fix
• New feature
• Performance improvement
• Refactoring
• Documentation update
• Other

Is this a breaking change?

• Yes
• No

How Has This Been Tested?

• cargo fmt --all -- --check
• cargo test sinks::util::service::health::tests::health_config_default_matches_deserialize_defaults --lib
• cargo test sinks::util::service::health::tests::test_health_counters --lib
• ./scripts/check_changelog_fragments.sh

Checklist

• I have read the contributing guidelines
• I have added tests that prove my fix is effective or that my feature works
• I have added an appropriate changelog entry

[pront]

Thanks!