[CODE HEALTH] Fix clang-tidy misc-override-with-different-visibility warnings

[thc1006]

Contributes to #4195

Changes

Fixes a subset of #4195 by aligning the visibility of virtual function overrides with their base-class declaration, which is the fix the misc-override-with-different-visibility check asks for (per the check documentation).

This PR resolves 31 of the 36 reported warnings across 13 files. The remaining 5 warnings (in ext/include/opentelemetry/ext/http/server/http_server.h and socket_tools.h) arise from private/protected inheritance of SocketTools::Reactor::SocketCallback and need a different, more careful change; they are deferred to a follow-up PR to keep this one reviewable.

Fix pattern

• Base public -> derived override made public (widening; e.g. test mocks of GetLogger/GetTracer, Predicate::Match, Fields)
• Base protected -> derived override made protected (e.g. sdk::logs::Logger::EnabledImplementation, gtest SetUp/TearDown)
• Base private (NVI) -> derived override made private (e.g. MetricReader::OnForceFlush/OnShutDown/OnInitialized)

No behavior change: only access specifiers on overrides are adjusted; .cc definitions do not restate visibility.

Verification

• Built all affected test and example targets locally (clang-22, ABI v1 and v2 presets); all compile and link.
• clang-format clean on all changed files.
• The warning_limit in .github/workflows/clang-tidy.yaml is lowered accordingly (abiv1: 309 -> 280, abiv2: 319 -> 288), derived from the current CI baseline minus the 29 (abiv1) / 31 (abiv2) warnings resolved here.

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 82.83%. Comparing base (db83a82) to head (d340a9f).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #4215   +/-   ##
=======================================
  Coverage   82.83%   82.83%           
=======================================
  Files         415      415           
  Lines       17431    17431           
=======================================
  Hits        14437    14437           
  Misses       2994     2994           

Files with missing lines	Coverage Δ
...entelemetry/trace/propagation/http_trace_context.h	92.86% <ø> (ø)
...plar/aligned_histogram_bucket_exemplar_reservoir.h	86.67% <ø> (ø)
...include/opentelemetry/sdk/metrics/view/predicate.h	91.67% <ø> (ø)

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[marcalff]

LGTM, thanks for the cleanup