`UnusedVariable` behaves inconsistently for method references and equivalent lambda expressions.

### Error Prone version

2.49.0 (error_prone_core)

### Description

`UnusedVariable` behaves inconsistently for method references and equivalent lambda expressions. When `transform(String name)` is called via `this::transform`, Error Prone does not report `name` as unused. After rewriting the call site to `n -> transform(n)`, the same parameter is flagged. Since the method body is unchanged and `name` is never used, the method-reference case is a false negative.

### Reproducer

```java
package demo;

import java.util.List;
import java.util.stream.Collectors;

public class TypeParameterMethodRef {

// BEFORE: no UnusedVariable warning on `name`
public List> processRef(List names) {
return names.stream()
.map(this::transform)
.collect(Collectors.toList());
}

// AFTER: UnusedVariable reported on `name`
public List> processLambda(List names) {
return names.stream()
.map(n -> transform(n))
.collect(Collectors.toList());
}

private List transform(String name) {
throw new UnsupportedOperationException("transform not implemented");
}
}
```

### Actual behavior

`UnusedVariable` is reported only when `transform` is called from a lambda; the method-reference call site suppresses the warning for the same unused parameter.