Fix: Multi-Selection Batch Operations with Unit Test Compatibility

[Szy-Cathay]

Fix: Multi-Selection Batch Operations with Unit Test Compatibility

Description

This PR reintroduces batch delete and move operations for multi-selected tags in the Metadata Editor, with critical fixes to ensure full unit test compatibility and defensive programming practices.

Key Features:

• ✅ Multi-selection support via IsSelected property
• ✅ Backward compatibility with single-selection via SelectedTag
• ✅ Comprehensive unit tests covering all scenarios
• ✅ Defensive null/boundary checks for headless environments

Technical Rationale (Why This Approach?)

Problem Diagnosis

The previous implementation failed unit tests due to tight coupling between UI state and data operations:

1. Selection State Mismatch: Tests set SelectedTag property, but commands only checked IsSelected flags
2. No Fallback Mechanism: When IsSelected was empty, commands returned early instead of checking SelectedTag
3. Insufficient Defensive Programming: Lacked null checks for headless test environments

Solution Architecture

Adapter Pattern for Selection Modes:

┌─────────────────────────────────────────┐
│ Input: IsSelected (multi) OR SelectedTag │
└──────────────┬──────────────────────────┘
│ Priority: IsSelected > SelectedTag
▼
┌─────────────────────────────────────────┐
│ Unified: List │
└──────────────┬──────────────────────────┘
│ Pure data operations
▼
┌─────────────────────────────────────────┐
│ Output: Batch delete/move operations │
└─────────────────────────────────────────┘

Key Design Decisions:

1. Dual-Mode Selection Handler (GetSelectedItems method):

   • First checks IsSelected for multi-selection scenarios
   • Falls back to SelectedTag for single-selection backward compatibility
   • Returns empty list if neither is set (safe no-op)

2. Defensive Programming Practices:

   • Null checks on controller and ParsedFile before any operation
   • Boundary validation (e.g., can't move top item up, bottom item down)
   • Safe collection modification using ToList() snapshot pattern

3. Group Movement Logic:

   • Selected items move as a cohesive block
   • Sorted traversal order prevents items from "jumping over" each other
   • Selection state automatically restored after move operations

Test Rationale

Test Coverage Strategy:

Test Scenario	Purpose
DeleteAction_MultiSelectedTags	Verify batch deletion via IsSelected

Why These Tests Matter:

• Regression Prevention: Existing tests (MetaDataEditor_DeleteAndSave, MetaDataEditor_MoveUpAndSave) continue to pass
• New Feature Validation: Multi-selection scenarios are fully covered
• Edge Case Handling: Null states, boundaries, and empty selections are tested
• Integration Verification: Save/reload cycle confirms data persistence

Changes Summary

Modified Files:

• DeleteEntryCommand.cs: Added dual-mode selection, null safety, improved documentation
• MoveEntryCommand.cs: Added dual-mode selection, boundary checks, selection restoration

New Test File:

• MetaDataEditorMultiSelectTests.cs: 11 comprehensive test cases

Verification Steps

1. Run existing tests: dotnet test --filter "FullyQualifiedName~MetaDataEditorViewModelTests"
2. Run new tests: dotnet test --filter "FullyQualifiedName~MetaDataEditorMultiSelectTests"
3. All 17 tests (6 existing + 11 new) should pass ✅

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Technical Rationale Summary: This approach prioritizes test compatibility, backward compatibility, and defensive programming. The dual-mode selection handler ensures that both existing test patterns (SelectedTag) and new multi-selection features (IsSelected) work seamlessly, while comprehensive null/boundary checks prevent crashes in headless environments. The additional unit tests provide confidence that the implementation is robust and maintainable.