Refactor: Use TryParse in view models for safer numeric input parsing

[dhtzs]

Summary of the Pull Request

Replaces parsing logic with TryParse across view models to prevent overflow/format crashes from invalid user input. Specifically fixes the VM Idle Timeout crash reported in #14312, while improving stability for other numeric settings. Also removes an orphaned reference to SystemdSettingsExpander in OptionalFeaturesPage.xaml.cs which caused a Debug.Assert failure and crashed the application when the Optional Features page was opened. This expander is no longer present in the XAML markup.

PR Checklist

• Closes: WSL Settings crash after putting too large number in VM Idle Timeout #14312
• Communication: I've discussed this with core contributors already. If work hasn't been agreed, this work might be rejected
• Tests: Added/updated if needed and all pass
• Localization: All end user facing strings can be localized
• Dev docs: Added/updated if needed
• Documentation updated: If checked, please file a pull request on our docs repo and link it here: #xxx

Detailed Description of the Pull Request / Additional comments

Validation Steps Performed

[Copilot]

Pull request overview

This PR hardens numeric settings input handling in the WSL Settings app by replacing exception-throwing numeric conversions with TryParse in several settings view models, preventing crashes from overflow/invalid user input (notably the VM Idle Timeout crash in #14312).

Changes:

• Replace Convert.ToInt32/ToUInt64 with Int32.TryParse / UInt64.TryParse in multiple numeric-setting setters.
• On parse failure (e.g., overflow), revert the UI by raising OnPropertyChanged() instead of crashing.
• Apply the same safer parsing pattern to optional features, networking, memory/processor, and filesystem settings.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated no comments.

File	Description
src/windows/wslsettings/ViewModels/Settings/OptionalFeaturesViewModel.cs	Uses Int32.TryParse for VM Idle Timeout to avoid overflow crashes and revert invalid input.
src/windows/wslsettings/ViewModels/Settings/NetworkingViewModel.cs	Uses Int32.TryParse for Initial Auto Proxy Timeout to prevent invalid input crashes.
src/windows/wslsettings/ViewModels/Settings/MemAndProcViewModel.cs	Uses TryParse for processor count, memory size, and swap size numeric inputs to prevent exceptions.
src/windows/wslsettings/ViewModels/Settings/FileSystemViewModel.cs	Uses UInt64.TryParse for Default VHD Size to avoid invalid/overflow conversion crashes.

[benhillis]

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