Add unit tests for SystemTray.Linux (0% → 80%+ coverage)

[Olbrasoft]

Summary

SystemTray.Linux has no unit tests. Test project exists but contains only empty placeholder test.

Problem

• Current state: 0% test coverage
• Impact: No safety net for refactoring, regressions not caught, violates quality standards
• Target: 80%+ coverage for business logic (per refactoring guide)

Proposed Solution

Add comprehensive unit tests for all public APIs:

Priority 1 - Core functionality (Target: 90%+)

• TrayIconManager - create/get/remove icons
• TrayIcon - initialize, set icon, show/hide, dispose
• IconRenderer - render, cache, pre-cache

Priority 2 - Animation & State (Target: 85%+)

• TrayIcon.StartAnimation / StopAnimation
• TrayIcon.SetIcon with different states
• Icon state transitions

Priority 3 - Internal (Target: 70%+)

• StatusNotifierItemHandler - D-Bus message handling

Test Strategy

• Use Moq for ILogger, IIconRenderer mocking
• Integration tests for D-Bus communication (may need separate project)
• Test error handling (null params, disposed objects, file not found)

Files Affected

• tests/SystemTray.Linux.Tests/ - add new test files:
  • TrayIconManagerTests.cs
  • TrayIconTests.cs
  • IconRendererTests.cs
  • StatusNotifierItemHandlerTests.cs

Acceptance Criteria

• 80%+ code coverage for business logic
• All public methods have tests
• Edge cases covered (null, disposed, errors)
• Tests follow naming convention: MethodName_Scenario_ExpectedResult
• Remove placeholder UnitTest1.cs