chore(tracing-otel): inline HeadersTextMap{Getter,Setter} into OtelTracer

[eschizoid]

Summary

• Both HeadersTextMapGetter and HeadersTextMapSetter were package-private SPI adapters with exactly one production callsite (OtelTracer itself), zero tests, zero examples. Their Javadoc already declared them as "Used by OtelTracer#..." — conceptually private helpers leaking out as top-level types.
• Inlined both as private static final class nested inside OtelTracer (kept as parallel nested classes rather than mixing a lambda for the setter and a class for the getter — TextMapGetter has two methods so it can't be a lambda anyway, and the symmetric shape reads better).
• Preserved the stateless-singleton INSTANCE semantics from the originals; callsites were updated from HeadersTextMapGetter.INSTANCE / HeadersTextMapSetter.INSTANCE to the nested HeadersGetter.INSTANCE / HeadersSetter.INSTANCE.
• Dropped the [HeadersTextMapSetter] / [HeadersTextMapGetter] bullet from package-info.java and folded the propagation note into the OtelTracer bullet.

Cohesion win

• The adapters are now invisible outside OtelTracer — they cannot be referenced from anywhere else in the module, which matches their actual usage. One fewer top-level surface for kpipe-tracing-otel to defend.
• Two fewer compilation units in the module (3 → 1 production class).
• LOC delta: -61 / +53 = -8 lines including the Javadoc carry-over and the new imports for TextMapGetter / TextMapSetter / Header / ArrayList / StandardCharsets in OtelTracer.

JPMS

• module-info.java only exports io.github.eschizoid.kpipe.tracing.otel; (whole package). The deleted classes were package-private, so they were never independently visible. No module-info edit needed.

Test plan

• ./gradlew :lib:kpipe-tracing-otel:test → BUILD SUCCESSFUL
• grep -rn 'HeadersTextMapGetter\|HeadersTextMapSetter' lib/ examples/ benchmarks/ --include='*.java' --include='*.kt' returns no hits

[eschizoid]

@copilot please review

[codecov]

Codecov Report

❌ Patch coverage is 60.00000% with 6 lines in your changes missing coverage. Please review.
✅ Project coverage is 77.82%. Comparing base (4ccac98) to head (cb9d5e9).
⚠️ Report is 5 commits behind head on main.
✅ All tests successful. No failed tests found.

Files with missing lines	Patch %	Lines
...ithub/eschizoid/kpipe/tracing/otel/OtelTracer.java	60.00%	3 Missing and 3 partials ⚠️

Additional details and impacted files

@@            Coverage Diff            @@
##               main     #198   +/-   ##
=========================================
  Coverage     77.82%   77.82%           
+ Complexity      732      727    -5     
=========================================
  Files            68       66    -2     
  Lines          2800     2800           
  Branches        346      348    +2     
=========================================
  Hits           2179     2179           
  Misses          462      462           
  Partials        159      159           

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