feat: add stampTraceHeaders kafka helper

[delthas]

Summary

Add kafka.stampTraceHeaders(entry) to the tracing module: it returns a copy of a node-rdkafka entry with the active span's trace headers attached, or the entry unchanged when tracing is off / no span is active (so the OTEL-off path keeps its original message shape — no headers: undefined).

This replaces the lower-level traceHeadersFromCurrentContext in the public API: that function is now an internal helper, since every consumer was using it only to stamp headers onto an entry. Folding the if (headers) entry.headers = headers boilerplate into one helper removes that repetition at the call sites (backbeat has ~4).

traceHeadersFromEntry, contextFromKafkaHeaders, and startLinkedSpanFromKafkaEntry are unchanged.

Issue: ARSN-586

[bert-e]

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[bert-e]

Waiting for approval

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 73.94%. Comparing base (5944736) to head (a4a20c6).

Additional details and impacted files

@@                 Coverage Diff                 @@
##           development/8.4    #2635      +/-   ##
===================================================
+ Coverage            73.92%   73.94%   +0.02%     
===================================================
  Files                  229      229              
  Lines                18480    18483       +3     
  Branches              3847     3848       +1     
===================================================
+ Hits                 13661    13667       +6     
+ Misses                4814     4811       -3     
  Partials                 5        5              

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[claude]

• Removing the traceHeadersFromCurrentContext export is a breaking API change, but the version bump is patch (8.4.5 → 8.4.6) instead of at least minor
  - Bump to 8.5.0, or keep the old function exported (deprecated) alongside the new stampTraceHeaders so downstream consumers aren't broken
  
  Review by Claude Code