JUnit tests for the language server #129

Why: the server maps verifier output to diagnostics and decides which folder to verify. PathUtils.extractBasePath cuts the path at the first src folder plus one more, so the folder layout decides what gets verified. This logic is deterministic and cheap to test.

What:

• Add server/src/test/java, with tests for PathUtils (including a src folder at different depths, and Windows-style URIs) and the DTO/diagnostic converters.
• Add mvn -B test to test.yml.

References:

• Eclipse LemMinX (a Java language server used by VS Code's XML extension) keeps JUnit tests for its services next to the server: https://github.com/eclipse/lemminx/tree/main/org.eclipse.lemminx/src/test/java/org/eclipse/lemminx