Function handle_webhook 

pub async fn handle_webhook(
    path: &str,
    body: &[u8],
    headers: &HeaderMap,
    source_addr: Option<String>,
    host: &EmbeddedHost,
    webhook_registry: &RwLock<WebhookRegistry>,
) -> Result<(FlowRunId, Value), WebhookError>

Handle an incoming webhook invocation.

Looks up the registered webhook, builds trigger input from the request, submits the flow with trigger data injected into ContextStore, and returns the FlowRunId and receipt.