Fixes #5237. Fix Markdown table row height calculation

[Copilot]

Fixes

• Fixes Markdown - Table row calculations incorrect #5237

Initial layout of a Markdown view with a large table produces extra blank lines after the table. Resizing the terminal corrects the layout.

Proposed Changes/Todos

• Fix table height placeholder calculation in BuildRenderedLines

Root Cause

Add(tableView) triggers EndInit() → Layout() → OnSubViewLayout → Recalculate(Frame.Width) using a stale content width from the previous layout pass. This overwrites RenderedHeight with an incorrect value before it's read for placeholder line reservation.

Fix

Capture RenderedHeight immediately after Recalculate(tableLayoutWidth) and before Add(tableView):

tableView.Recalculate (tableLayoutWidth);

// Capture BEFORE Add() — Add triggers EndInit → Layout → Recalculate at stale width
int tableHeight = tableView.RenderedHeight;

// ... event wiring ...

_tableViews.Add (tableView);
Add (tableView);

for (var i = 0; i < tableHeight; i++)
{
    _renderedLines.Add (...);
}

Previously tableView.RenderedHeight was read after Add(tableView), by which point the value had been corrupted by the initialization cascade.

Pull Request checklist:

• I've named my PR in the form of "Fixes #issue. Terse description."
• My code follows the style guidelines of Terminal.Gui - if you use Visual Studio, hit CTRL-K-D to automatically reformat your files before committing.
• My code follows the Terminal.Gui library design guidelines
• I ran dotnet test before commit
• I have made corresponding changes to the API documentation (using /// style comments)
• My changes generate no new warnings
• I have checked my code and corrected any poor grammar or misspellings
• I conducted basic QA to assure all features are working