fix: initialize ProfileManagementPageCreationContext in OnPostAsync of ManageModel

[maliming]

ManageModel.OnPostAsync returned Page() without initializing ProfileManagementPageCreationContext, causing a NullReferenceException when the view tried to render Model.ProfileManagementPageCreationContext.Groups inside the <abp-tabs> tag helper.

Although normal usage relies on JavaScript to intercept form submissions (using AJAX instead of a native HTML POST), a direct POST to /Account/Manage — triggered by JS being disabled, a JS load failure, or an automated tool — would hit OnPostAsync and crash during view rendering.

The fix mirrors the initialization logic from OnGetAsync: creating a new ProfileManagementPageCreationContext and running all registered contributors through it before returning Page().

[Copilot]

Pull request overview

This PR updates the Account “Manage” Razor Page model so that POST requests rebuild the ProfileManagementPageCreationContext and run all configured ProfileManagementPageOptions.Contributors, matching the GET behavior and ensuring the page can render correctly after a POST.

Changes:

• Make OnPostAsync asynchronous.
• Initialize ProfileManagementPageCreationContext on POST and run contributors via ConfigureAsync.