fix(consumer): metricize Kafka transport failures

[onewland]

Summary

• intercept Arroyo global BrokerTransportFailure events before Sentry sends them
• increment rust_consumer.kafka_error with error=broker_transport_failure instead
• drop the duplicate Sentry structured log while preserving local JSON output
• preserve all other Kafka errors and add focused classifier coverage

This addresses SNUBA-AA1, a recoverable librdkafka broker connection failure that produced 744 events over the inspected 24-hour period.

Tests

• cargo test logging::tests
• cargo clippy --all-targets -- -D warnings
• cargo fmt --check
• commit hooks