kafka: emit arrangement key only when a user key encoder exists

The previous refactor conflated two distinct conditions under a single
key_is_synthetic flag: whether any key exists (user or relation) for PK
violation detection, and whether a user-configured key should be emitted to
the Kafka message. The old zip_into_diff_pairs stripped the emitted key
whenever user_key_indices.is_none() — covering both the relation-key case
and the synthetic-hash case. My refactor only stripped when BOTH were
absent, so for sinks with a relation key but no user key, the arrangement
key leaked through to encode_collection. The key_encoder is None in that
case (no user key_format was configured), and the expect("key present")
at line 1537 panicked.

Fix: gate the emitted key on key_encoder.is_some() instead of
!key_is_synthetic. Matches the old behavior exactly and is a local
decision the sink can make from its own config without a trait change.

Author: DAlperin

src/storage/src/sink/kafka.rs

@@ -1496,7 +1496,10 @@ fn encode_collection<'scope>(
                             if let Some(warner) = pk_warner.as_mut() {
                                 warner.observe(key, time);
                             }
-                            let key_for_message = if key_is_synthetic { &None } else { key };
+                            // Only emit the arrangement key when the user configured one; relation-key
+                            // and synthetic-hash arrangements exist purely for grouping / worker
+                            // distribution and have no corresponding key encoder.
+                            let key_for_message = if key_encoder.is_some() { key } else { &None };
 
                             let mut hash = None;
                             let mut headers = vec![];