TestSourceJarMojo forks to generate-sources instead of generate-test-sources

`TestSourceJarMojo.java:28` has `@Execute(phase = "generate-sources")`:

```java
@Mojo(name = "test-jar", defaultPhase = "package")
@Execute(phase = "generate-sources")
public class TestSourceJarMojo extends TestSourceJarNoForkMojo {
```

This is a test-jar goal but forks only to `generate-sources`, not `generate-test-sources`. If test sources are generated by another plugin in the `generate-test-sources` phase, they will not be available when the mojo runs, leading to an empty or incomplete test sources JAR.

Compare with `TestSourceGeneratedJarMojo.java:28` which correctly uses `generate-test-sources`:

```java
@Mojo(name = "generated-test-jar", defaultPhase = "package")
@Execute(phase = "generate-test-sources")
public class TestSourceGeneratedJarMojo extends TestSourceJarNoForkMojo {
```

**Fix**: Change the phase to `generate-test-sources`.