[CODE HEALTH] Move func_grpc_main classes into anonymous namespace

[thc1006]

Summary

Wraps the 3 file-scope class/struct declarations in
functional/otlp/func_grpc_main.cc within an anonymous namespace,
clearing the misc-use-internal-linkage warnings.

7th PR of the misc-use-internal-linkage campaign and the twin of
#4128 (func_http_main, merged). Same wrap pattern, same boundary
rationale, same +3 site clearance.

Changes

• functional/otlp/func_grpc_main.cc: insert namespace { after
  the command-line opt_* variables and } // namespace after the
  test_case struct closing brace. Wrap covers TestResult (line
  63), TestLogHandler (line 128), and test_case (line 316).
• .github/workflows/clang-tidy.yaml: abiv1 349 -> 346 (-3),
  abiv2 353 -> 350 (-3).
• CHANGELOG.md: one bullet under [Unreleased].

Why the wrap ends before line 322

Same rationale as #4128: the static int test_*() forward
declarations are matched by definitions in the lower half of the
file (after main()). Wrapping declarations alone would produce
(anonymous namespace)::test_* symbols that fail to link.

main(), the static const test_case all_tests[] table,
list_test_cases, run_test_case, and the test definitions remain
at file scope.

Test plan

• Local build of func_otlp_grpc: 122/122 ninja steps
  (-DWITH_OTLP_HTTP=ON -DWITH_OTLP_GRPC=ON per the CMake quirk
  that the outer functional/CMakeLists.txt only adds otlp subdir
  when WITH_OTLP_HTTP=ON)
• func_otlp_grpc --help smoke test prints usage
• 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 81.94%. Comparing base (5791b30) to head (5cb0ddf).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #4129   +/-   ##
=======================================
  Coverage   81.94%   81.94%           
=======================================
  Files         385      385           
  Lines       16067    16067           
=======================================
  Hits        13164    13164           
  Misses       2903     2903           

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