[BUG] Mock plugin fails requests when httpStatusCode is missing or invalid

### 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-plugin

### What happened

`MockPlugin` assumes every cached mock rule has a valid `httpStatusCode`:

```java
MockHandle mockHandle = MockPluginHandler.CACHED_HANDLE.get().obtainHandle(CacheKeyUtils.INST.getKey(rule));
...
exchange.getResponse().setStatusCode(HttpStatus.valueOf(mockHandle.getHttpStatusCode()));
```

But `MockHandle.httpStatusCode` is nullable and has no model-level default:

```java
private Integer httpStatusCode;
```

The admin plugin handle metadata also marks this field as not required while only providing a UI/default metadata value:

```sql
INSERT INTO plugin_handle ... 'httpStatusCode' ... '{"required":"0","defaultValue":"200","rule":""}'
```

So a rule synced or saved with a missing `httpStatusCode` produces a `NullPointerException` from unboxing in `HttpStatus.valueOf(...)`. An out-of-range value such as `999` produces `IllegalArgumentException`. In both cases the mock rule cannot return its configured response and the request fails with a gateway error instead of a controlled mock response or a validation error when the rule is created.

### Expected behavior

Mock rules should not be able to crash request processing because of an absent/invalid status code. Either:

- validate `httpStatusCode` as required and within a valid HTTP status range when the rule is saved/synced, or
- default missing values to `200` and reject/normalize invalid values before calling `HttpStatus.valueOf(...)`.

### How to reproduce

1. Enable the mock plugin.
2. Create or sync a mock rule whose handle omits `httpStatusCode`, for example:

```json
{"responseContent":"{\"ok\":true}"}
```

or uses an invalid code:

```json
{"httpStatusCode":999,"responseContent":"{\"ok\":true}"}
```

3. Send a request matching the mock rule.
4. The request fails because `MockPlugin` calls `HttpStatus.valueOf(mockHandle.getHttpStatusCode())` without a null/range check.

### Debug logs

_No response_

### Environment

Current `master` branch.

### Are you willing to submit a PR?

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

Start in MockPlugin at the call to HttpStatus.valueOf(mockHandle.getHttpStatusCode()), then trace MockHandle and MockPluginHandler.CACHED_HANDLE to see where mock rules are saved or synced. Check the plugin_handle metadata and reproduce with a missing or 999 status; done means request processing no longer crashes and invalid values are handled before this call.

Written by the indexing model from the issue text.