[CODE HEALTH] Move remaining misc-use-internal-linkage sites into anonymous namespaces

[om7057]

Summary

Closes the last four sites left open on #4196 after the earlier batch of PRs (#4199, #4200, #4217, #4225, #4231, #4286, #4301, #4302, #4303):

• exporters/otlp/src/otlp_file_client.cc: OtlpFileSystemBackend and OtlpFileOstreamBackend extended into the file's existing anonymous namespace
• exporters/elasticsearch/src/es_log_record_exporter.cc: ResponseHandler and AsyncResponseHandler wrapped in an anonymous namespace
• exporters/elasticsearch/src/es_log_recordable.cc: json_assign_visitor wrapped in an anonymous namespace nested inside namespace nlohmann
• functional/otlp/func_grpc_main.cc / func_http_main.cc: the TestMode / test_mode enums (declared before the file's existing anonymous namespace block, since the static variable using them is referenced throughout the whole translation unit) each given their own small anonymous-namespace wrap

All four are file-local classes/enums with no header exposure, so this is a mechanical, low-risk change with no behavior difference.

Test plan

• opentelemetry_exporter_otlp_file_client builds clean (WITH_OTLP_FILE=ON)
• opentelemetry_exporter_elasticsearch_logs builds clean (WITH_ELASTICSEARCH=ON), confirmed via object-file timestamps newer than the source edits
• functional/otlp/func_http_main.cc compiles clean via direct compiler invocation (WITH_OTLP_HTTP=ON)
• functional/otlp/func_grpc_main.cc syntax-checked clean (-fsyntax-only) using the project's real include paths (gRPC dependency build was skipped locally to avoid a slow from-source clone; the file itself doesn't depend on gRPC headers)

Fixes #4196

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 80.85%. Comparing base (e79ab1f) to head (320d2d3).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #4350   +/-   ##
=======================================
  Coverage   80.85%   80.85%           
=======================================
  Files         450      450           
  Lines       19215    19215           
=======================================
  Hits        15535    15535           
  Misses       3680     3680           

Files with missing lines	Coverage Δ
...orters/elasticsearch/src/es_log_record_exporter.cc	12.22% <ø> (ø)
exporters/elasticsearch/src/es_log_recordable.cc	65.39% <ø> (ø)
exporters/otlp/src/otlp_file_client.cc	70.38% <ø> (ø)

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[dbarker]

Looks good. Thanks.