[Rework] MIME detection via Lua Magic; enforce cfg in Lua task API

[Rework] MIME detection via Lua Magic; enforce cfg in Lua task API

- Add rspamd_mime_parser_config on cfg; remove global state and lazy init
- Initialize parser config once per cfg; preload lua_magic.detect_mime_part
- Always run detection after normal part parse; promote .eml/message parts
- Preserve detected_ext/detected_ct/detected_type and NO_TEXT flag
- Remove duplicate detection from message.c; add debug logs
- Restore CTE parsing API and fix call sites
- Enforce cfg requirement in rspamd_task.load_from_string/load_from_file/create
- Fix unit tests to pass rspamd_config to load_from_string