feat: Add EditForm.SubmitAsync for programmatic form submission

[JayArrowz]

Add EditForm.SubmitAsync for programmatic form submission

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Description

Currently there is no way to submit an EditForm programmatically from C# code, because HandleSubmitAsync is private. This forces developers to manually call EditContext.Validate() and then invoke their submit handler separately, which breaks the form's submission flow and bypasses any client-side validation prompts.

This PR adds a public SubmitAsync method to EditForm that submits the form from code, triggering the same validation and submission lifecycle as if the user had clicked a submit button.

As suggested by @SteveSandersonMS #48613 (comment), this works by making a JS interop call to instruct the browser to submit the form via HTMLFormElement.requestSubmit(). This ensures that any expected client-side behaviors, such as triggering HTML's built-in validation prompts, also occur.

Interactive rendering: The resulting submit event is handled by the form's onsubmit handler, which runs the EditContext validation and invokes the appropriate OnSubmit, OnValidSubmit, or OnInvalidSubmit callback.

Server-side rendering (SSR): Not applicable. SubmitAsync relies on JS interop, which requires an interactive rendering mode. In pure SSR scenarios there is no JS interop available to call this method, use a regular instead.

Usage:

<EditForm Model="@model" OnValidSubmit="@HandleValidSubmit" @ref="_editForm">
    <DataAnnotationsValidator />
    <InputText @bind-Value="model.Name" />
</EditForm>

<button @onclick="SubmitFromCode">Submit</button>

@code {
    private EditForm _editForm = null!;

    private async Task SubmitFromCode()
        => await _editForm.SubmitAsync();
}

Fixes #48613

[JayArrowz]

@dotnet-policy-service agree

[dotnet-policy-service]

Looks like this PR hasn't been active for some time and the codebase could have been changed in the meantime.
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