Move the embedded HTTP server headers out of ext/include

ext/include/opentelemetry/ext/http/server/http_server.h and socket_tools.h are an embedded HTTP server used by tests and by the http example. They sit under ext/include beside the user facing client headers, which makes them look like part of the public surface even though nothing supported points at them.

#4327 replaced the *.h glob in the ext_common component with an explicit manifest and left both out, so the package no longer installs them. That fixes the packaging side. The files are still in a public looking location, and @dbarker asked for a follow up so the move itself does not get lost.

What uses them today

• ext/test/http/socket_tools_test.cc
• ext/test/http/curl_http_test.cc
• ext/test/w3c_tracecontext_http_test_server/main.cc
• exporters/otlp/test/otlp_http_exporter_test.cc
• exporters/otlp/test/otlp_http_log_record_exporter_test.cc
• exporters/zipkin/test/zipkin_exporter_test.cc
• examples/http/server.h

Everything except the example is a test. That list is the full transitive closure over the include graph on main, so nothing under functional/ is involved.

What a move would settle

Each of those targets now links ws2_32 on Windows for itself, because #4327 took that link off the opentelemetry_ext interface once the headers stopped shipping. On the Bazel side //ext:headers still globs the whole include tree, so its -DEFAULTLIB:Ws2_32.lib linkopt is still load bearing and stays. Moving the files gives both build systems the same story: a server target that owns its own Winsock dependency, and an ext target that is only the HTTP client.

test_common is one candidate, though the http example is not a test, so a small dedicated target may fit better. Happy to take this on once there is a preferred destination.

Start by tracing the includes from ext/test/http/socket_tools_test.cc, the listed exporter tests, and examples/http/server.h, then inspect the test_common and ext targets. The destination is not yet settled: compare test_common with a dedicated server target. Done means the headers move out of ext/include, all listed targets still build, and Winsock ownership remains correct for both build systems.