axonflow-sdk-java: Webhook signing-key (HMAC-SHA256) not exposed by SDK type, preventing signature verification

Summary

## Summary

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.

## Affected versions

Versions prior to 7.0.0.

## Impact

A webhook receiver using the SDK's typed API to handle inbound deliveries had no path to authenticate the source of incoming payloads. An attacker who learned the webhook URL — through misconfiguration, log leakage, observable network traffic during setup, or any other discovery channel — could forge webhook deliveries indistinguishable from legitimate ones, causing the receiving application to act on fabricated events (e.g. simulated approval-granted callbacks, simulated policy-decision callbacks, simulated step-completion callbacks).

## Remediation

Upgrade to the patched version listed in Vulnerabilities below. The signing key is now exposed on the `WebhookSubscription` response type returned by `CreateWebhook`. Implementations should:

1. Persist the secret returned by `CreateWebhook` securely (it is only returned once, at create time). 2. On each incoming webhook delivery, compute `HMAC-SHA256(secret, raw_body)` and compare it in constant time against the `X-AxonFlow-Signature` header. 3. Reject any delivery whose signature does not match.

## Credit

Identified by AxonFlow internal security review during the April 2026 quality-freeze epic.