Add CTabFolder test coverage for untested API and edge cases

[vogella]

Summary

• Adds 13 focused tests for CTabFolder covering previously untested behavior
• Targets edge cases and behavioral contracts, not trivial getter/setter round-trips
• Contributes to CTabFolder: Improve readability, performance, simplification, and test coverage #3219 (Section 4: Test Coverage Improvements)

Tests added

• Error handling: invalid arguments for setTabPosition and setTabHeight
• Single mode: verifies only selected tab shows in single mode
• Empty folder: operations on 0-item folder don't throw; state correct after disposing all items
• Selection boundaries: out-of-range index is a silent no-op; empty folder returns null/-1
• Min/max mutual exclusion: setMaximized(true) clears minimized state
• Tab compression: tabs shrink when shell is resized smaller
• SelectionListener contract: programmatic setSelection does NOT fire listener
• Item disposal: disposing the currently selected item doesn't crash
• showSelection: selected item is actually visible after call
• Hit-testing: getItem(Point) returns correct item or null

Test plan

• All 119 tests pass locally (0 failures, 0 errors)
• CI passes on all platforms

🤖 Generated with Claude Code

[github-actions]

Test Results

  170 files  ± 0    170 suites  ±0   27m 22s ⏱️ +10s
4 692 tests +13  4 671 ✅ +13   21 💤 ±0  0 ❌ ±0 
6 670 runs  +78  6 515 ✅ +78  155 💤 ±0  0 ❌ ±0 

Results for commit c632c59. ± Comparison against base commit 16e501f.

♻️ This comment has been updated with latest results.