Function run_callback_handler 

pub async fn run_callback_handler(
    callback_rx: Receiver<CallbackRequest>,
    callbacks_map: Arc<HashMap<String, Arc<dyn Callback>>>,
    resource_limits: ResourceLimits,
    secrets: Arc<HashMap<String, SecretConfig>>,
) -> u32

Handle callback requests with concurrent execution.

Uses tokio::select! to concurrently:

1. Receive new callback requests from the channel
2. Poll in-flight callback futures to completion

This allows multiple callbacks to execute in parallel when Python code uses asyncio.gather() or similar patterns.

Returns the total number of callback invocations.