Replace unit-style specs with end-to-end integration tests #497

Current specs only verify LSP attach/capabilities against a single empty buffer.

Replace them with proper integration tests that:

• launch real Neovim headless against a real Maven Java project fixture
• verify JDTLS attaches and the project imports correctly
• exercise plugin features end-to-end: extensions bundling, diagnostics, DAP config, built-in runner (JavaRunnerRunMain/StopMain/ToggleLogs), test runner (JavaTestRunCurrentClass/Method/AllTests, report viewer), profile UI, settings runtime
• run in CI on all supported platforms