GHSA-mph8-9v29-pm42: axonflow-sdk-typescript: Webhook signing-key (HMAC-SHA256) not exposed by SDK type, preventing signature verification

The AxonFlow SDK’s WebhookSubscription (or equivalent) type did not expose the HMAC-SHA256 signing key returned by the platform’s CreateWebhook endpoint. Without access to the secret through the typed SDK API, callers had no path to verify the X-AxonFlow-Signature header on incoming webhook deliveries. Affected callers had two unsatisfactory options:

1. Skip signature verification entirely — accepting any payload from any source that knew the webhook URL.
2. Hand-parse the raw HTTP JSON response to extract the secret, bypassing the type-safe SDK surface.

This advisory is filed across all four AxonFlow SDKs (Go, Python, TypeScript, Java) because the same defect and the same fix landed in each.