Unused empty capture group in regex pattern

## Description

In `MappingUtils.java:90`, the `RegexBasedInterpolator` is constructed with:

```java
new RegexBasedInterpolator("\\@\\{(", ")?([^}]+)\\}@");
```

The prefix contains an opening `(` and the suffix starts with `)?`, forming an empty optional capture group `()?` that spans across the interpolator's internal key capture pattern. This group is never used for anything and is functionally dead regex.

The same result could be achieved with the simpler:

```java
new RegexBasedInterpolator("\\@\\{", "\\}@");
```

## Impact

No functional impact, but the unnecessary complexity obscures the intent and may confuse future contributors.

## Relevant code

`src/main/java/org/apache/maven/shared/mapping/MappingUtils.java:90`