[BUG] Apollo registry ignores modified instance events and keeps stale cache

### Current Behavior

The Apollo registry implementation updates its local instance cache for added and deleted keys, but ignores modified keys.

`ApolloInstanceRegisterRepository.selectInstances()` registers a `ConfigChangeListener` and handles only `ADDED` and `DELETED` events:

```java
switch (changeEvent.getChange(key).getChangeType()) {
case ADDED:
childrenList.put(key, changeEvent.getChange(key).getNewValue());
break;
case DELETED:
childrenList.remove(key);
break;
default:
break;
}
```

Apollo change events can also be `MODIFIED`. When an existing instance key is updated, for example host, port, scheme, or other instance JSON fields change while the key remains the same, the local `childrenList` is not updated.

### Expected Behavior

`MODIFIED` events should refresh the cached value for that instance key, the same as an add/update cache write.

### Impact

Consumers of `selectInstances()` can keep using stale instance data until the key is deleted/recreated or the process restarts and reloads the full Apollo state.

### Code Location

- `shenyu-registry/shenyu-registry-apollo/src/main/java/org/apache/shenyu/registry/apollo/ApolloInstanceRegisterRepository.java`
- `selectInstances()` listener ignores modified Apollo property changes.

Open shenyu-registry/shenyu-registry-apollo/src/main/java/org/apache/shenyu/registry/apollo/ApolloInstanceRegisterRepository.java and start at selectInstances(). Trace the ConfigChangeListener handling for ADDED, DELETED, and MODIFIED events. Done means a modified Apollo instance refreshes the corresponding cached value, with the relevant registry module tests passing.

Written by the indexing model from the issue text.