Fixes #5099 - Preserve filename on FileDialog table navigation

[Copilot]

Proposed Changes/Todos

• Keep PreserveFilenameOnDirectoryChanges intact when TableView navigation triggers an internal PushState.
  • Guard TableViewOnValueChanged while _pushingState is active so table refreshes do not overwrite the typed filename with the first file in the newly opened directory.
• Add a regression test for the reported SaveDialog scenario with an active file-type filter.
  • Verifies Enter on a directory row preserves the filename.
  • Verifies the following Enter on .. navigates back to the parent directory instead of closing the dialog.

var ofd = new SaveDialog ()
{
    AllowedTypes = [new AllowedType ("C# File", ".cs")],
    Path = "hello.cs",
};
ofd.Style.PreserveFilenameOnDirectoryChanges = true;

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

[Copilot]

Pull request overview

Updates FileDialog’s TableView navigation to avoid overwriting a user-typed filename when directory changes trigger an internal PushState, particularly when PreserveFilenameOnDirectoryChanges is enabled.

Changes:

• Guard TableViewOnValueChanged with _pushingState to prevent re-entrant/refresh-driven selection changes from rewriting Path.
• Add an integration regression test covering “Enter on directory” and then “Enter on ..” navigation while preserving the typed filename.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 comment.

File	Description
Tests/IntegrationTests/FluentTests/FileDialogTests.cs	Adds a regression integration test for SaveDialog TableView Enter navigation + filename preservation.
Terminal.Gui/Views/FileDialogs/FileDialog.TableView.cs	Prevents ValueChanged side effects during internal state pushes by checking _pushingState.

[tig]

This needs @tznind's approval since he reported it.

[tznind]

It would be good if the copilot first committed test so we could see failure then did fix in followup commit to see it fixed with no further test change. Even if the pr were then squashed

Looks good though.