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>

include/nlohmann/detail/output/serializer.hpp

@@ -730,12 +730,17 @@ class serializer
         write_buffer_pos += length;
     }
 
+  JSON_PRIVATE_UNLESS_TESTED:
     /*!
     @brief flush the write buffer to the output adapter
 
     Writing zero characters is a well-defined no-op for every output adapter, so
     the buffered length is passed through unconditionally (no empty-guard branch
     to leave uncovered).
+
+    @note dump_escaped() and dump_integer()/dump_float() write into the internal
+    write buffer; callers that invoke them directly (rather than through the
+    public dump()) must call flush() before inspecting the output.
     */
     void flush()
     {
@@ -743,6 +748,7 @@ class serializer
         write_buffer_pos = 0;
     }
 
+  private:
     /*!
     @brief count digits