Fix Table BoxStyle.Heavy body separators and add validator tests

[tedd]

This commit updates the Unicode Box Drawing characters used for body separation when a Table control is configured with BoxStyle.Heavy. Previously, it incorrectly used mixed light/heavy characters. The change aligns it with strict heavy lines (\u2523, \u252B, \u254B, \u2501).

It also introduces a new exhaustive unit test Table_CoordinatePreciseCharacterAssertion_HeavyBodySeparation in ValidatorTableTests.cs to ensure coordinate-precise mapping and robust behavior during dynamic hierarchical grid resizing. A failing coverage test was also fixed.

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PR created automatically by Jules for task 3537628775893930386 started by @tedd

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