test(producer): pin KafkaMessageSink error paths

[eschizoid]

Summary

Closes a criticality-6 coverage gap on KafkaMessageSink: the existing tests only covered happy paths. This PR adds three tests that pin the error-path contract — no production code changes.

Invariants pinned

• acceptPropagatesProducerExceptionAsRuntime — when the underlying Kafka Producer.send(record, callback) throws a KafkaException synchronously (serialization failure, authorization failure, buffer exhaustion with max.block.ms=0, etc.), the sink must propagate the exception unwrapped, not catch-and-log. The async-callback failure path is the only "log and swallow" path the sink documents (per the class javadoc); synchronous throws are surfaced so upstream sink error handling (MessageProcessorRegistry.withSinkErrorHandling) can see them. Pins §12 (no-silent-failures doctrine).

• acceptOnClosedProducerThrows — per Kafka's Producer contract, calling send(...) on a closed producer throws IllegalStateException("Cannot perform operation after producer has been closed"). The sink does not pre-check producer state and does not swallow this exception — it propagates so pipeline error handlers can route or fail loudly.

• ofStaticFactoryWithNullKeyAccepted — the KafkaMessageSink.of(producer, topic, valueMapper, tracer) factory produces a sink whose key path is null. Verifies the resulting ProducerRecord has a null key (Kafka allows null keys; partition assignment falls back to the configured partitioner — round-robin / sticky for the default).

Test plan

• ./gradlew :lib:kpipe-producer:test — 7 tests in KafkaMessageSinkTest all green
• Full producer module test suite — green
• No production code changes — pure test addition

[eschizoid]

@copilot please review

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 77.60%. Comparing base (eb21492) to head (b4ddf67).
⚠️ Report is 4 commits behind head on main.
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@             Coverage Diff              @@
##               main     #192      +/-   ##
============================================
- Coverage     77.64%   77.60%   -0.04%     
+ Complexity      732      731       -1     
============================================
  Files            68       68              
  Lines          2800     2800              
  Branches        346      346              
============================================
- Hits           2174     2173       -1     
  Misses          465      465              
- Partials        161      162       +1     

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[eschizoid]

@copilot please review