Fix FlexBox pref/min size computation for wrapping

[FlorianKirmaier]

Summary

• Fixed computePrefHeight(width) and computePrefWidth(height) to simulate wrap line-breaking when the available dimension is known, instead of always reporting single-row/column sizes
• Added getContentBias() override so JavaFX parent layouts pass the actual width/height to size computations
• Fixed computeMinHeight(width) and computeMinWidth(height) with the same wrapping logic
• Added test app boundary elements (right + bottom) to visually verify FlexBox respects its container bounds
• Added 23 unit tests covering prefSize wrapping, minSize wrapping, contentBias, and single-element cases

Test plan

• All existing FlexBox tests pass
• New FlexBoxPrefSizeTest tests pass (23 tests)
• Verify visually in FlexBoxTestApp that the FlexBox no longer overflows into boundary elements

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