[CODE HEALTH] Move remaining API test helpers into anonymous namespaces

[thc1006]

Part of #4196

Draft while CI runs. Ready for review from my side.

Six misc-use-internal-linkage sites in the API tests, continuing from #4217 and #4286. This is the first of three batches; the other two cover the SDK tests.

file	classes
api/test/logs/logger_test.cc	TestLogger, TestProvider
api/test/logs/provider_test.cc	TestProvider, TestEventLoggerProvider
api/test/trace/propagation/detail/string_test.cc	SplitStringTestFixture
api/test/trace/provider_test.cc	TestProvider

Shape of the change

logger_test.cc and string_test.cc already had an anonymous namespace that stopped just short of the flagged classes, so those namespaces are extended rather than joined by a second one. In logger_test.cc that means moving the opening up past TestLogger and the closing down to the end of the file, since TestProvider sat below it.

The two provider_test.cc files get a namespace around the test body. In the logs one that places the deprecated-declaration pragma push and pop pair inside the namespace, which is fine because both ends stay on the same side of the boundary. Both ABI settings compile, which exercises that.

Worth flagging: api/test/logs contains two different classes both named TestProvider, one in logger_test.cc and one in provider_test.cc. They currently build into separate test executables (a distinct Bazel cc_test and a distinct CMake target each), so there is no present link-time conflict; being in the same Bazel package does not put them in the same binary. Giving them internal linkage nevertheless prevents an ODR hazard if these sources are ever combined into one test binary.

Warning limit

151 to 145 on abiv1, and 161 to 156 on abiv2. The abiv2 delta is five rather than six because TestEventLoggerProvider sits behind deprecated-API pragmas and is only reported on abiv1. That single site is also the entire difference between the 25 and 24 totals I posted on #4196. If the artifact disagrees with those numbers I will correct them from it.

Verification

• All six classes are inside an anonymous namespace after the change, checked by walking the namespace depth rather than by eye, and every file has balanced open and close.
• All four files compile with -DOPENTELEMETRY_ABI_VERSION_NO=1 and =2.
• clang-format 18 clean.
• No TEST, TEST_F, or TEST_P suite name that would map to different fixture types is shared across translation units in any combined test target. string_test.cc uses TEST_P/INSTANTIATE_TEST_SUITE_P, and its fixture and parameter type are both in the anonymous namespace now; neither api/test/logs nor api/test/trace has a combined test target. So this should not repeat the all_tests redefinition that [CODE HEALTH] Move api/test and sdk/test classes into anonymous namespace #4217 ran into, which was the main thing I wanted to rule out before sending this.

One honest gap: I could not re-run the check itself locally. misc-use-internal-linkage only reports classes from llvm-22 onward, which is what CI uses; clang-tidy 18, 20 and 21 report functions and variables only, so a local run returns zero both before and after the change and proves nothing. I checked that rather than assuming it. The site list therefore comes from the CI log artifact for main at de8fa515, and CI is the authority on the resulting count.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 81.30%. Comparing base (d3c7a47) to head (9b588cd).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4301      +/-   ##
==========================================
- Coverage   81.31%   81.30%   -0.01%     
==========================================
  Files         445      445              
  Lines       18859    18859              
==========================================
- Hits        15333    15331       -2     
- Misses       3526     3528       +2     

see 1 file with indirect coverage changes

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