Issue webhook doesn't fire when assignee is removed

### Environment SaaS ([https://sentry.io/](<https://sentry.io/>)) ### Steps to Reproduce 1. Create an integration with issue webhooks enabled 2. Assign a user to an issue in Sentry 3. Observe webhook fires with action `assigned` 4. Remove the assignee from the same issue 5. Check webhook logs ### Expected Result Webhook fires with action `assigned` (with `assignedTo: null`) or a new `unassigned` action, allowing integrations to track when issues become unassigned. ### Actual Result No webhook is sent when an issue is unassigned. Integrations cannot detect unassignment without polling. ### Product Area Settings - Integrations We want to define a set of [lightweight metrics](<https://docs.sentry.io/product/explore/metrics/>) that SDKs can automatically collect for e.g. framework, language or platform, focusing only on signals that **do NOT overlap with Tracing**. Ideally these metrics should give a quick sense of app or runtime health, … while deeper investigation will still rely on traces or profiles.<br><br>Examples**:** * Node.js runtime metrics (e.g., event loop delay, heap usage) * Prisma-related metrics in Next.js * Mobile CPU / memory usage to help surface potential ANRs or app hangs --- **Candidate Metrics (add more below)** * … *(please also add ideas, questions or examples in comments)*