Use grapheme iteration in DrawHotString

[akrantz]

DrawHotString iterated by rune via EnumerateRunes and rendered each rune individually with AddRune. This breaks grapheme clusters: combining marks sent separately via AddRune fail to compose with their base character (e.g. e + combining acute renders as two separate characters instead of 'é').

Replace with GraphemeHelper.GetGraphemes iteration and AddStr, which preserves grapheme cluster integrity. The hotkey specifier check is unchanged — it only matches single-character graphemes.

Added tests:

• DrawHotString_CombiningMarks: 3 cases verifying combining marks compose correctly (acute accent, tilde, embedded in word)
• DrawHotString_HotkeyWithCombiningMarks: hotkey + combining marks

Fixes

• DrawHotString iterates by rune instead of grapheme, breaking combining marks #4800

Proposed Changes/Todos

• Replace EnumerateRunes() with GraphemeHelper.GetGraphemes() in DrawHotString
• Replace AddRune(rune) with AddStr(grapheme) to preserve grapheme cluster integrity
• Add tests for combining marks rendering (acute accent, tilde, embedded in word)
• Add test for hotkey specifier combined with combining marks

Pull Request checklist:

• I've named my PR in the form of "Fixes #issue. Terse description."
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• My code follows the Terminal.Gui library design guidelines
• I ran dotnet test before commit
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• 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

[tig]

Nice work! See my request.

[tig]

Another huge thank you!