[BUG] NamespacePluginServiceImpl.create never publishes a sync event; new namespace-plugin relation never reaches the gateway

## Description
`create` inserts `NamespacePluginRelDO` and returns, but never calls `namespacePluginEventPublisher.onCreated(vo)`. The publisher's `onCreated` (line 63) publishes both `DataChangedEvent(PLUGIN, CREATE)` for gateway sync and a `NamespacePluginCreatedEvent` for audit — it is simply never invoked from `create`. Every other mutating method in this service publishes (`update`->`onUpdated`, `delete`->`onDeleted`, `enabled`->`onEnabled`).

## Location
```
shenyu-admin/.../service/impl/NamespacePluginServiceImpl.java:94-103
namespacePluginEventPublisher.onCreated at NamespacePluginEventPublisher.java:63
```

## Impact
A plugin added to a namespace via `POST /namespace-plugin/{namespaceId}/{pluginId}` is saved in the admin DB but never pushed to the bootstrap/gateway. The plugin is not active in that namespace until a manual `syncPluginAll`/`syncPluginData` is invoked. No audit log entry is created either.

## Suggested fix
After `insertSelective`, fetch the VO via `selectByPluginIdAndNamespaceId(pluginId, namespaceId)` and call `namespacePluginEventPublisher.onCreated(vo)`.

## Related existing issue(s)
None

_Identified during the 2026-08-02 audit; full list in [`docs/issue-candidates-2026-08-02.md`](docs/issue-candidates-2026-08-02.md)._

Start in shenyu-admin/.../service/impl/NamespacePluginServiceImpl.java at lines 94-103 and compare create with the other mutating methods. Read NamespacePluginEventPublisher.java at line 63 and verify that creation fetches the relation VO before publishing the event. Done means namespace plugin creation triggers gateway synchronization and the audit event without requiring manual sync.

Written by the indexing model from the issue text.