[Test] Add tests for /checkv3 multipart endpoint

[Test] Add tests for /checkv3 multipart endpoint

C++ unit tests (23 cases): multipart form parser, response builder,
and round-trip serialization. Robot Framework functional tests (6 cases):
GTUBE scan, metadata handling, settings_id, and error cases for missing
parts and malformed boundaries. Python helpers for building and parsing
multipart/form-data requests and multipart/mixed responses.