[BUG] AI proxy plugin-level config is not applied at runtime

### Current Behavior

The enhanced AI proxy runtime still tries to resolve a global `AiCommonConfig` from `Singleton.INST`, but the plugin data handler never stores plugin-level configuration there.

`AiProxyConfigService.resolvePrimaryConfig()` reads and merges global config:

```java
final AiCommonConfig globalConfig = Singleton.INST.get(AiCommonConfig.class);
...
if (Objects.nonNull(globalConfig)) {
primaryConfig.mergeWith(globalConfig);
}
```

However, `AiProxyPluginHandler.handlerPlugin()` intentionally performs no plugin config handling:

```java
// No global plugin configuration handling is performed.
```

As a result, plugin-level/global AI proxy defaults configured in admin are not applied at runtime. If a selector relies on global defaults for required fields such as provider/model/baseUrl/apiKey, `AiProxyPlugin.createChatModel()` later fails because the resolved config is incomplete.

### Expected Behavior

Either plugin-level AI proxy configuration should be loaded into the runtime config path used by `resolvePrimaryConfig()`, or the runtime should stop advertising/depending on a global config merge path.

### Impact

Deployments that configure shared AI proxy defaults at plugin/global level can fail requests unless every selector repeats a fully populated AI provider configuration.

### Code Location

- `shenyu-plugin/shenyu-plugin-ai/shenyu-plugin-ai-proxy/src/main/java/org/apache/shenyu/plugin/ai/proxy/enhanced/service/AiProxyConfigService.java`
- `resolvePrimaryConfig()` reads `Singleton.INST.get(AiCommonConfig.class)`.
- `shenyu-plugin/shenyu-plugin-ai/shenyu-plugin-ai-proxy/src/main/java/org/apache/shenyu/plugin/ai/proxy/enhanced/handler/AiProxyPluginHandler.java`
- `handlerPlugin()` does not populate global plugin configuration.
- `shenyu-plugin/shenyu-plugin-ai/shenyu-plugin-ai-proxy/src/main/java/org/apache/shenyu/plugin/ai/proxy/enhanced/AiProxyPlugin.java`
- `createChatModel()` hard-fails when required provider/factory config is missing.

Read AiProxyConfigService.resolvePrimaryConfig(), AiProxyPluginHandler.handlerPlugin(), and AiProxyPlugin.createChatModel() to trace how plugin-level configuration reaches runtime resolution. Determine which of the two expected approaches fits the existing flow. Done means shared AI proxy defaults configured at plugin/global level are applied, or the runtime no longer depends on that merge path, without incomplete provider configuration failures.

Written by the indexing model from the issue text.