Function handle_completion 

pub async fn handle_completion(
    state: Arc<ServerState>,
    params: CompletionParams,
    client: Client,
    config: Arc<RwLock<DepsConfig>>,
) -> Option<CompletionResponse>

Handles completion requests.

Delegates to the appropriate ecosystem implementation based on the document type. Falls back to text-based completion when TOML parsing fails (user is still typing).