Add unit and integration tests

[nam-hle]

Summary

Add test coverage for core plugin logic. The codebase currently has zero tests.

Motivation

As the plugin grows, regressions become likely. Tests for the foundational utilities and scanners provide a safety net and enable confident refactoring.

Acceptance Criteria

• NadleFileUtil tests: config file pattern matching, task name extraction, regex edge cases
• NadleTaskScanner tests: single file scanning, monorepo prefix generation, skip-directory logic
• NodeJsResolver tests: fallback chain behavior, path resolution order
• NadleTaskGotoDeclarationHandler tests: PSI navigation for dependsOn references
• CI runs tests on every PR (if CI exists)

Implementation Notes

• Use IntelliJ Platform test framework (BasePlatformTestCase or LightPlatformCodeInsightTestCase)
• Place tests in src/test/kotlin/ mirroring the main source structure
• For PSI-dependent tests, use test fixtures with sample nadle.config.ts files
• Pure logic tests (file util, scanner) can use plain JUnit