Fixes #5087 - Removes Dialog scrolling code and and fixes other bugs along the way (#5090)

* WIP: Diagnosing #5087 and fixing other bugs along the way

* Refactor FileDialog types, filters, and code style

- Moved AllowedTypeAny and IAllowedType to separate files
- Enhanced Notepad open dialog filters and options
- Replaced Enumerable.Empty<T>() with [] literals
- Changed CancelButton to CancelButtonIndex (int)
- Moved FileDialog UI/filter setup to EndInit
- Simplified FileDialogState and FileDialogTableSource
- Updated OpenDialog/SaveDialog to use CancelButtonIndex
- Applied code style and expression-bodied member improvements

* Remove subview min size logic from Dialog<TResult>

Eliminates tracking and usage of _minimumSubViewsSize. Dialog sizing now only considers viewport, title, and button sizes. OnSubViewAdded override and related calculations are removed for simplification.

* Update Terminal.Gui/Views/FileDialogs/FileDialog.Commands.cs

Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>

* Update Examples/UICatalog/Scenarios/Notepad.cs

Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>

* Fix nullable string warning in AutocompleteFilepathContext by using OfType&lt;string&gt;() after Where filter

Agent-Logs-Url: https://github.com/gui-cs/Terminal.Gui/sessions/30418e61-6cfd-44e9-a7f7-0c239168714e

Co-authored-by: tig <585482+tig@users.noreply.github.com>

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Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>
Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>
Co-authored-by: tig <585482+tig@users.noreply.github.com>

Author: 3 people