fix: detect JUnit 5 from submodule build files and default to JUnit 5

[mashraf-222]

Problem

Multi-module Gradle projects (OpenRewrite, Spring, etc.) were misdetected as JUnit 4 projects. The AI service then generated JUnit 4 tests that failed to compile because the project only has JUnit 5 dependencies.

Root Cause

_detect_test_deps_from_gradle() in config.py only checked the root build.gradle(.kts). For multi-module projects, the root build file typically only has plugin declarations — JUnit dependencies are in module-level build files.

Secondary: the default framework was junit4, which is wrong for the vast majority of modern Java projects.

Fix

1. After checking the root build file, scan immediate child directories for build.gradle(.kts) files
2. Changed the default framework from junit4 to junit5 (JUnit 5 has been the standard since 2017)

Test Coverage

• 3 new tests in TestGradleSubmoduleDetection:
  • test_detect_gradle_junit5_from_submodule: Multi-module with junit-jupiter in submodule only
  • test_detect_gradle_junit5_from_root: Single-module with junit-jupiter in root (regression check)
  • test_default_to_junit5_when_nothing_detected: Empty project defaults to junit5

Testing

$ uv run pytest tests/test_languages/test_java/test_config.py::TestGradleSubmoduleDetection -v
3 passed

All 21 existing config tests still pass (18 existing + 3 new).