[CODE HEALTH] Move api/test and sdk/test classes into anonymous namespace

[thc1006]

Changes

Part of #4196 (and #2053). Wraps file-local test helper classes in an anonymous namespace to enforce internal linkage, resolving the misc-use-internal-linkage warnings across 20 files under api/test and sdk/test. This follows the same approach as #4199 and #4200.

To keep the review manageable, this is the first batch. The remaining files (under exporters, ext, and functional, plus a few that need manual handling due to existing or nested namespaces) will follow in later batches. The files that overlap with my other open code-health PRs (#4215, #4216) are also deferred to avoid conflicts.

Four metrics tests that share the WritableMetricStorageTestFixture fixture (async_metric_storage_test and the sync_metric_storage counter/gauge/up_down_counter tests) are also deferred. They are linked together into //sdk/test/metrics:all_tests, and moving same-named fixtures into per-file anonymous namespaces makes gtest treat them as a redefinition of one test suite. They will be handled in a follow-up that renames the fixtures or shares one via a header.

Verification

• Compiles clean (clang-22, abiv2 preset); a local clang-tidy run confirms the 31 warnings on these 20 files are resolved with no new warnings, and the code still builds.
• clang-format clean on all changed files.
• warning_limit lowered from 309/319 to 278/288 (the current baseline minus the 31 resolved here).

Note: this touches the same warning_limit line as #4215 and #4216. Whichever of these merges first, I will rebase the others accordingly (the final limit after all three is lower).

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[marcalff]

CI failure, to investigate:

FAIL: //sdk/test/metrics:all_tests (Aborted) (see /home/runner/.cache/bazel/e6b8d6759295e14b76bc8cb98b604748/execroot/_main/bazel-out/k8-fastbuild/testlogs/sdk/test/metrics/all_tests/test.log)
INFO: From Testing //sdk/test/metrics:all_tests:
==================== Test output for //sdk/test/metrics:all_tests:

[ ERROR ] external/googletest+/googletest/src/gtest.cc:2838:: sdk/test/metrics/sync_metric_storage_counter_test.cc:43: Attempted redefinition of test suite WritableMetricStorageTestFixture.
All tests in the same test suite must use the same test fixture
class.  However, in test suite WritableMetricStorageTestFixture, you tried
to define a test using a fixture class different from the one
used earlier. This can happen if the two fixture classes are
from different namespaces and have the same name. You should
probably rename one of the classes to put the tests into different
test suites.```

[thc1006]

Thanks for flagging this, fixed and pushed.

Those four files (async_metric_storage_test and the sync_metric_storage counter/gauge/up_down_counter ones) all declare WritableMetricStorageTestFixture at namespace scope. That was fine while they shared a single external-linkage class, but once each moved into its own anonymous namespace they became four different classes sharing one suite name, and gtest aborts on that when //sdk/test/metrics:all_tests links them together.

Renaming as the gtest message suggests would work, but a test-suite rename feels out of place in a plain internal-linkage cleanup, so I've put those four back to their state on main and left them out of this batch. That takes the PR down to 20 files with the limit at 278/288. I'll pick the four up separately, either renaming the fixtures or hoisting a shared one into a header, whichever you'd rather see.

I didn't catch this locally because I was verifying with the CMake clang-tidy build, which links each test into its own binary and never exercises the combined target. I've added a duplicate-fixture-name check across the batch so it won't slip through again.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 82.83%. Comparing base (4d09399) to head (e7074ee).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #4217   +/-   ##
=======================================
  Coverage   82.83%   82.83%           
=======================================
  Files         415      415           
  Lines       17423    17423           
=======================================
  Hits        14431    14431           
  Misses       2992     2992           

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[marcalff]

LGTM, thanks for the fix.