[BUG] Response cryptor returns plaintext when the configured field is missing

### Current Behavior

The response cryptor plugin returns the original response body unchanged when the configured field path is missing.

For field mapping, `MapTypeEnum.FIELD.convert()` returns `null` when the configured field cannot be parsed from the response body:

```java
String parseBody = JsonUtil.parser(originalBody, ruleHandle.getFieldNames());
if (Objects.isNull(parseBody)) {
return null;
}
```

`AbstractCryptorPlugin.convert()` treats `null` as a field-parse error and delegates to `fieldErrorParse(...)`. For response cryptor, `CryptorResponsePlugin.fieldErrorParse()` returns `originalBody` unchanged.

### Expected Behavior

When response encryption/decryption is configured but the target field cannot be found, the plugin should fail closed or return a clear cryptor configuration error instead of silently returning the unmodified body.

### Impact

If an upstream response shape changes or a configured field path is wrong, response cryptor can silently bypass the configured response transformation and send plaintext/unencrypted data downstream.

### Code Location

- `shenyu-plugin/shenyu-plugin-security/shenyu-plugin-cryptor/src/main/java/org/apache/shenyu/plugin/cryptor/strategy/MapTypeEnum.java`
- `shenyu-plugin/shenyu-plugin-security/shenyu-plugin-cryptor/src/main/java/org/apache/shenyu/plugin/cryptor/plugin/AbstractCryptorPlugin.java`
- `shenyu-plugin/shenyu-plugin-security/shenyu-plugin-cryptor/src/main/java/org/apache/shenyu/plugin/cryptor/plugin/CryptorResponsePlugin.java`

Start with MapTypeEnum.java and trace the null result through AbstractCryptorPlugin.java into CryptorResponsePlugin.java. Reproduce the missing-field case and inspect existing cryptor error handling before choosing the expected failure behavior. Done means a missing configured field cannot return the original response body and is covered by a regression test.

Written by the indexing model from the issue text.