Change produce_sync to return DeliveryHandle for API consistency

[mensfeld]

Summary

Currently, produce_sync is the only produce method that returns a DeliveryReport instead of a DeliveryHandle. All other methods consistently return handles:

Method	Returns
produce_async	DeliveryHandle
produce_many_async	Array<DeliveryHandle>
produce_sync	DeliveryReport ← inconsistent
produce_many_sync	Array<DeliveryHandle>
flush_async	Array<DeliveryHandle>
flush_sync	Array<DeliveryHandle>

Proposal

Change produce_sync to return DeliveryHandle (in its final state) instead of DeliveryReport for full API consistency.

Users who need the report can call handle.create_result.

Breaking Change

This is a breaking change. Code like:

report = producer.produce_sync(topic: 'test', payload: 'data')
puts report.offset

Would need to become:

handle = producer.produce_sync(topic: 'test', payload: 'data')
puts handle.create_result.offset
# or
puts handle.offset  # if we delegate offset to the handle

Context

This was identified while fixing documentation inconsistencies in #525 follow-up. The produce_many_sync method was intentionally changed to return handles (instead of reports) in #525 to ensure consistent types in both success and error flows. Applying the same pattern to produce_sync would complete the API consistency.