[runtime] send Kafka tombstones on state pruning for durable log compaction

[rob-9]

Addresses #691

Purpose of change

KafkaActionStateStore.pruneState() only removed entries from the in-memory cache. the underlying Kafka topic was never updated, so:

1. the topic grew unbounded, meaning Kafka log compaction had no tombstones to trigger key deletion
2. after a restart, rebuildState() replayed pruned records back into memory, resurrecting state that should have been discarded

This PR sends null-valued records to the state topic during pruning, and updates rebuildState() to skip tombstoned keys during replay instead of inserting them into the cache. Also fixes an NPE in the error-logging path that called record.value().toString() on a null value.

Tests

mvn --batch-mode --no-transfer-progress -pl runtime -am -Dtest=KafkaActionStateStoreTest test

• testPruneState verifies tombstones are produced and in-memory entries are evicted
• testPruneStateSendsTombstonesWithCorrectKeys asserts exact composite keys on produced tombstones
• testPruneStateNoMatchingKeys verifies no tombstones sent when no keys match
• testPruneStateWithNullProducer verifies graceful degradation when producer is null

API

No public API changes.

Documentation

• doc-not-needed