[JUnit Platform Engine] Warn when selected line does not exist

[mpkorstanje]

⚡️ What's your motivation?

It is possible to select a line in a classpath resource that does not exist. For example:

@Suite
@IncludeEngines("cucumber")
@SelectClasspathResource(value = "path/to/example.feature", line = 9001)
public class RunCucumberTest {
}

In a trivial suite this will result in an exception as no scenarios could be found. In a non-trivial suite that does discover other scenarios this error will go unnoticed.

By reporting this as a discovery issue and falling back to running the whole feature we ensure that no-tests go unexecuted while also informing the callers about this mis-configuration.

Example of the warning:

Aug 31, 2025 5:46:09 PM org.junit.platform.launcher.core.DiscoveryIssueNotifier logIssues
WARNING: TestEngine with ID 'junit-platform-suite' encountered a non-critical issue during test discovery:

(1) [WARNING] Feature file classpath:path/to/example.feature does not have a feature, rule, scenario, or example element at line 9001. Selecting the whole feature instead

🏷️ What kind of change is this?

• 🐛 Bug fix (non-breaking change which fixes a defect)

📋 Checklist:

• I agree to respect and uphold the Cucumber Community Code of Conduct
• I've changed the behaviour of the code
  • I have added/updated tests to cover my changes.
• My change requires a change to the documentation.
  • I have updated the documentation accordingly.
• Users should know about my change
  • I have added an entry to the "Unreleased" section of the CHANGELOG, linking to this pull request.