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

[thc1006]

Changes

Completes #4195 by resolving the 5 misc-override-with-different-visibility warnings that #4215 deferred, all in the ext/http embedded server. Each override had wider visibility than its inheritance-adjusted base:

• HttpServer privately inherits SocketCallback, so its onSocketAcceptable, onSocketReadable, onSocketWritable and onSocketClosed overrides are aligned from protected to private.
• Reactor inherits Thread as protected, so its onThread override is aligned from public to protected.

Scope and compatibility

These overrides are dispatched only through their base interfaces (SocketCallback& and Thread::onThread), so no dynamic-dispatch call path changes. The change does narrow the source-level surface a derived class could reach (for example, calling the base implementation from a subclass) in these installed headers.

I chose this direction because the private inheritance of SocketCallback looks deliberate (the callbacks are HttpServer's internal reactor plumbing) and HttpServer is extended through its public request-handler API (e.g. operator[], setServerName) rather than by overriding the reactor callbacks; FileHttpServer is such a subclass and does not touch them. If maintainers would rather preserve that extension surface, two alternatives are: switch HttpServer to protected inheritance of SocketCallback (keeps the callbacks protected but widens the base-conversion surface), or NOLINT the four overrides. I am happy to take whichever direction you prefer.

The unrelated sendMore() helper keeps its protected visibility (an earlier revision narrowed it along with the section; that has been reverted).

Verification

• A syntax-only build (clang-18, C++14) of file_http_server.h, which pulls in http_server.h and socket_tools.h, the FileHttpServer subclass, and an HttpServer instantiation, compiles cleanly.
• clang-format and markdownlint 0.46.0 clean.
• warning_limit lowered from 163/173 to 158/168. I will confirm from the clang-tidy report artifacts that misc-override-with-different-visibility reaches 0 and the totals match before treating this as ready.

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 81.31%. Comparing base (de8fa51) to head (a2ce156).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #4280   +/-   ##
=======================================
  Coverage   81.31%   81.31%           
=======================================
  Files         445      445           
  Lines       18859    18859           
=======================================
  Hits        15333    15333           
  Misses       3526     3526           

Files with missing lines	Coverage Δ
...nclude/opentelemetry/ext/http/server/http_server.h	65.92% <ø> (ø)
...clude/opentelemetry/ext/http/server/socket_tools.h	94.00% <ø> (ø)

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[dbarker]

Looks good to me. Thanks.