Fix: move test only helpers, remove unnecessary lint suppressions

[swashtek]

move test only helpers, remove unnecessary lint suppressions public APIs

Change Summary

1. Move test only helpers behind #[cfg(test)], removes unnecessary #[allow(dead_code)]
2. Remove unnecessary lint suppressions on public APIs

What issue does this PR close?

Quick lint fixes for code hygiene

How are these changes tested?

Verified using cargo check and cargo test

• cargo check -p otap-df-channel
• cargo check -p otap-df-core-nodes
• cargo test -p otap-df-core-nodes -- syslog

Are there any user-facing changes?

No

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 88.17%. Comparing base (8a0e66a) to head (a75763a).
⚠️ Report is 3 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2773      +/-   ##
==========================================
- Coverage   88.18%   88.17%   -0.01%     
==========================================
  Files         647      647              
  Lines      250275   250275              
==========================================
- Hits       220693   220692       -1     
- Misses      29058    29059       +1     
  Partials      524      524              

Components	Coverage Δ
otap-dataflow	89.76% <100.00%> (-0.01%)	⬇️
query_abstraction	80.61% <ø> (ø)
query_engine	90.75% <ø> (ø)
otel-arrow-go	52.45% <ø> (ø)
quiver	92.25% <ø> (ø)

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[albertlockett]

Thanks @swashtek !