[CODE HEALTH] Move registry.cc propagator builders into anonymous namespace

[thc1006]

Summary

Wraps the 5 file-local TextMapPropagatorBuilder subclasses in
sdk/src/configuration/registry.cc within an anonymous namespace,
clearing the 5 misc-use-internal-linkage warnings clang-tidy reports
on this file. First production-code PR of the
misc-use-internal-linkage campaign opened by #4115 / #4116.

Changes

• sdk/src/configuration/registry.cc: insert namespace { after the
  namespace configuration { opening and } // namespace before
  Registry::Registry(). Wrap covers TraceContextBuilder,
  BaggageBuilder, B3Builder, B3MultiBuilder, and JaegerBuilder.
  The 5 classes are only instantiated in Registry::Registry() via
  std::make_unique and are not referenced from any other translation
  unit (verified by repository-wide grep).
• .github/workflows/clang-tidy.yaml: abiv1 371 -> 366 (-5),
  abiv2 377 -> 372 (-5).
• CHANGELOG.md: one bullet under [Unreleased].

Linkage rationale

Registry::Registry() and the other Registry:: member function
definitions remain at namespace scope outside the anonymous namespace,
because as members of class Registry (declared in registry.h) they
must keep external linkage to satisfy ODR. The anonymous namespace
contents are reachable from the member function bodies via the implicit
using-directive that hoists anonymous namespace symbols into the
enclosing configuration namespace.

Test plan

• Local build of opentelemetry_configuration,
  yaml_propagator_test, and yaml_test succeeded (109/109 ninja
  steps with -DWITH_CONFIGURATION=ON)
• yaml_propagator_test passes 9/9 (directly exercises the 5
  builders via YAML propagator configuration)
• yaml_test passes 50/50
• clang-format --dry-run --Werror clean on the modified file
• DCO signed off

Part of #2053

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 82.00%. Comparing base (d8cfe2f) to head (d6e7146).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #4121   +/-   ##
=======================================
  Coverage   82.00%   82.00%           
=======================================
  Files         385      385           
  Lines       16031    16031           
=======================================
  Hits        13144    13144           
  Misses       2887     2887           

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[marcalff]

Thanks for the fix.