Search videos within a WeChat channel

Returns WeChat Channels in-channel search: search a creator's published videos by keyword within that channel only. Send keyword (Keyword to search within this channel (1-100 chars)), username (Creator finder username (v2_…@finder format)); optional raw. This endpoint performs first-page retrieval only (no cursor parameter exposed, single-page result), but the response still carries next_cursor / continue_flag for ref.