[BUG] Deleting an unknown discovery id throws NPE

### Current Behavior

Deleting a discovery record by an unknown id can throw `NullPointerException`.

`DiscoveryController.delete()` accepts any `discoveryId` path value and delegates directly to `discoveryService.delete(discoveryId)`. In the service, `discoveryMapper.selectById(discoveryId)` can return `null`, but the next line dereferences it:

```java
DiscoveryDO discoveryDO = discoveryMapper.selectById(discoveryId);
DiscoveryProcessor discoveryProcessor = discoveryProcessorHolder.chooseProcessor(discoveryDO.getDiscoveryType());
```

### Expected Behavior

Deleting an unknown discovery id should return a controlled not-found/parameter error response instead of an internal server error.

### Steps to Reproduce

```http
DELETE /discovery/not-exist
```

If no discovery row exists for that id, `discoveryDO.getDiscoveryType()` can throw.

### Code Location

- `shenyu-admin/src/main/java/org/apache/shenyu/admin/controller/DiscoveryController.java`
- `shenyu-admin/src/main/java/org/apache/shenyu/admin/service/impl/DiscoveryServiceImpl.java`

Start with DiscoveryController.java and DiscoveryServiceImpl.java, then reproduce DELETE /discovery/not-exist to observe the current response. Check nearby handling for missing records and existing tests before making the behavior consistent. Done means an unknown discovery id returns a controlled not-found or parameter error instead of a NullPointerException.

Written by the indexing model from the issue text.