[CODE HEALTH] Move func_http_main classes into anonymous namespace

[thc1006]

Summary

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

6th PR of the misc-use-internal-linkage campaign (#4115, #4116, #4121,
#4122, #4124 already merged). First PR in the campaign touching the
functional/ tree.

Changes

• functional/otlp/func_http_main.cc: insert namespace { after the
  command-line opt_* variables and } // namespace after the
  test_case struct closing brace. Wrap covers TestResult (line 62),
  TestLogHandler (line 145), and test_case (line 333).
• .github/workflows/clang-tidy.yaml: abiv1 352 -> 349 (-3),
  abiv2 356 -> 353 (-3).
• CHANGELOG.md: one bullet under [Unreleased].

Why the wrap ends before line 339 (NOT before main())

The static int test_*() forward declarations (lines 339-440) are
matched by definitions in the lower half of the file (lines 1484-1882,
after main()). Wrapping declarations alone would produce
(anonymous namespace)::test_* symbols that do not link against the
file-scope static int test_*() definitions.

Verified locally: a wrap extending to line 453 (just before main())
produced 12+ undefined reference linker errors. The shortened wrap
ending at line 337 builds cleanly.

main() keeps external linkage. The static const test_case all_tests[] table and list_test_cases / run_test_case helpers
remain at file scope with their existing static linkage.

Test plan

• Local build of func_otlp_http: 121/121 ninja steps
  (-DWITH_OTLP_HTTP=ON -DWITH_OTLP_GRPC=OFF)
• func_otlp_http --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 (5cdcc38) to head (2cc3267).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4128      +/-   ##
==========================================
- Coverage   81.95%   81.94%   -0.01%     
==========================================
  Files         385      385              
  Lines       16067    16067              
==========================================
- Hits        13166    13164       -2     
- Misses       2901     2903       +2     

see 1 file with indirect coverage changes

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