[generator] Invalid default init value for integer enum properties

### Expected Behavior

```java
private MaxHours maxHours = MaxHours.NUMBER_48;
```

### Actual Behaviour

Generated Code
MaxHours.java: correct — constants use NUMBER_ prefix
```java
public enum MaxHours {
@JsonProperty("38") NUMBER_38(38),
@JsonProperty("39") NUMBER_39(39),
@JsonProperty("48") NUMBER_48(48),
...
}
```

MyModel.java: broken — default value uses _ prefix

```java
private MaxHours maxHours = MaxHours._48; // ❌ MaxHours._48 does not exist
```

### Steps To Reproduce

When an OpenAPI schema defines an integer enum with a default value, the generated field initializer references a non-existent enum constant. The enum class is generated using the `NUMBER_` prefix (e.g. `NUMBER_48`), but the default value expression uses an `_` prefix (e.g. `_48`), producing code that does not compile.

```yaml
openapi: 3.1.1
info:
title: Integer Enum Default Value Repro
version: 1.0.0
paths: {}
components:
schemas:
MaxHours:
type: integer
enum:
- 38
- 39
- 48
default: 48
MyModel:
type: object
properties:
maxHours:
$ref: "#/components/schemas/MaxHours"
```

```gradle
micronaut {
runtime("netty")
testRuntime("junit5")
openapi {
server(file("repro.openapi.yaml")) {
apiPackageName = "com.repro.api"
modelPackageName = "com.repro.api.model"
}
}
}
```

The same mismatch also occurs when `x-enum-varnames` is used to provide custom names (e.g. H48). The enum class correctly picks up the custom names, but the default value expression still uses the _-prefixed form.

### Environment Information

Gradle plugin 5.0.0

### Example Application

_No response_

### Version

5.0.0

Start with the provided repro.openapi.yaml and the generated MaxHours.java and MyModel.java output, using the Gradle OpenAPI configuration to reproduce the issue. Compare the enum constants with the generated default value expression, including the x-enum-varnames case. Done means generated code compiles and the default references the existing enum constant.

Written by the indexing model from the issue text.