Function post_thread 

pub async fn post_thread(
    client: &dyn XApiClient,
    tweets: &[String],
    media_ids: Option<&[Vec<String>]>,
) -> Result<Vec<String>, ToolkitError>

Post a thread (ordered sequence of tweets).

Validates all tweet lengths up front. Chains replies sequentially. On partial failure, returns ToolkitError::ThreadPartialFailure with the IDs of successfully posted tweets.