Flush serializer buffer in dump_escaped unit test

test-convenience failed (macOS finished first; the failure is
platform-independent) because check_escaped() calls the internal
serializer::dump_escaped() directly and then reads the output stream.
Since dump_escaped() now writes into the serializer's internal write
buffer, the bytes were still buffered and the stream was empty.

Expose flush() under JSON_PRIVATE_UNLESS_TESTED (same visibility as
dump_escaped) and flush in check_escaped() before inspecting the output.
Per-string flushing inside dump_escaped() was rejected on purpose: it
would defeat the buffering that makes object/array-heavy dumps faster.
Library behavior is unchanged (flush()'s body is identical; only its
access label moved).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01XAYM1qhSA2FDaDcGfPW3fG
Signed-off-by: Niels Lohmann <mail@nlohmann.me>

tests/src/unit-convenience.cpp

@@ -100,6 +100,7 @@ void check_escaped(const char* original, const char* escaped, const bool ensure_
     std::stringstream ss;
     json::serializer s(nlohmann::detail::output_adapter<char>(ss), ' ');
     s.dump_escaped(original, ensure_ascii);
+    s.flush(); // dump_escaped writes into the serializer's internal buffer
     CHECK(ss.str() == escaped);
 }
 } // namespace