[BUG] Nacos instance updates can emit added, deleted, and updated events together

### Search before asking

- [x] I had searched in the [issues](https://github.com/apache/shenyu/issues) and found no similar issues.

### Apache ShenYu Component

shenyu-registry

### What happened

`NacosInstanceRegisterRepository.compareInstances()` computes added and deleted instances with full `Instance.equals()` comparisons, then computes updated instances by matching `instanceId` and checking `!currentInstance.equals(previousInstance)`.

Current code in `shenyu-registry/shenyu-registry-nacos/src/main/java/org/apache/shenyu/registry/nacos/NacosInstanceRegisterRepository.java`:

```java
Set addedInstances = currentInstances.stream()
.filter(item -> !previousInstances.contains(item))
.collect(Collectors.toSet());

Set deletedInstances = previousInstances.stream()
.filter(item -> !currentInstances.contains(item))
.collect(Collectors.toSet());

Set updatedInstances = currentInstances.stream()
.filter(currentInstance -> Objects.nonNull(currentInstance.getInstanceId())
&& previousInstances.stream().anyMatch(previousInstance -> StringUtils.isNotBlank(previousInstance.getInstanceId())
&& currentInstance.getInstanceId().equals(previousInstance.getInstanceId())
&& !currentInstance.equals(previousInstance)))
.collect(Collectors.toSet());
```

ShenYu currently uses Nacos client `2.2.4`. In that version, `com.alibaba.nacos.api.naming.pojo.Instance.equals()` compares `toString()` values, so changes to health, enabled flag, weight, metadata, and other fields make two objects unequal even when they represent the same `instanceId`.

As a result, a property-only update for the same Nacos instance can be emitted as:

1. `ADDED`, because the changed current `Instance` is not contained in `previousInstances`.
2. `DELETED`, because the old `Instance` is not contained in `currentInstances`.
3. `UPDATED`, because the same `instanceId` exists and `equals()` is false.

Downstream discovery consumers can receive contradictory events for one logical update.

### What you expected to happen

For the same Nacos `instanceId`, a property-only change should emit one `UPDATED` event. Added and deleted detection should compare stable instance identity, not full mutable instance state.

### How to reproduce

1. Use Nacos discovery with ShenYu.
2. Register one instance and let `NacosInstanceRegisterRepository` store it in `watcherInstanceRegisterMap`.
3. Change only a mutable property of the same Nacos instance, such as weight or metadata, while keeping the same `instanceId`.
4. Trigger the Nacos listener and inspect events from `compareInstances()`.

### Debug logs

_No response_

### Environment

Current master branch.

### Are you willing to submit PR?

- [ ] Yes I am willing to submit a PR!

Start in shenyu-registry/shenyu-registry-nacos/src/main/java/org/apache/shenyu/registry/nacos/NacosInstanceRegisterRepository.java, focusing on compareInstances() and the Nacos Instance identity behavior described in the issue. Verify the listener path that consumes its event sets. Done means a property-only change for one instanceId produces only UPDATED, while genuine additions and removals still produce ADDED or DELETED.

Written by the indexing model from the issue text.