Issue webhook doesn't fire when assignee is removed

### Environment

SaaS ([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]() 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.

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

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**Candidate Metrics (add more below)**

* …

*(please also add ideas, questions or examples in comments)*

Start by reproducing the assignment and unassignment sequence in the issue and inspect the webhook logs for both actions. Done means an enabled issue webhook is emitted when an assignee is removed, carrying either assignedTo: null or a defined unassigned action.

Written by the indexing model from the issue text.