[Refactor] Use enum and pass lua_State through url_callback_data

[Refactor] Use enum and pass lua_State through url_callback_data

- Add enum rspamd_url_lua_filter_result for return values (ACCEPT/SUSPICIOUS/REJECT)
- Replace magic numbers (0/1/2) with enum constants throughout
- Add lua_state field to struct url_callback_data
- Pass lua_State through rspamd_url_find_multiple() chain
- Update all callers: task contexts pass task->cfg->lua_state, others pass NULL
- rspamd_url_trie_generic_callback_common now uses cb->lua_state
- More idiomatic C code with proper type safety
- lua_State now universally available where URL parsing happens