Added mailboxes as local data sources (#1022)

app/MindWork AI Studio/Tools/Validation/DependentFieldValidation.cs

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+using MudBlazor.Interfaces;
+
+namespace AIStudio.Tools.Validation;
+
+/// <summary>
+/// Gives form fields a fresh verdict when a field they are checked against changes.
+/// </summary>
+/// <remarks>
+/// A MudForm checks a field only when that very field changes. Where the rule of one field reads
+/// another one -- the required confidence level is checked against the embedding provider, and so
+/// are the token limits -- changing the other field leaves the verdict as it was, and an error which
+/// no longer holds stays on screen. Hooked to the FieldChanged event of the form, this checks the
+/// dependent fields again after every change. A state which is no field of the form, such as
+/// showing the expert settings, passes no changed field at all.
+/// </remarks>
+public static class DependentFieldValidation
+{
+    /// <summary>
+    /// Validates the dependent fields anew.
+    /// </summary>
+    /// <remarks>
+    /// The field which just changed is skipped, because the form checks it anyway. So is every
+    /// field the user has not reached yet and which shows no error: a dialog just opened must not
+    /// greet anybody with errors about fields they have not filled in.
+    /// <br/><br/>
+    /// A field checks the value it holds itself, and a value set in code reaches it only with the
+    /// next render. Where the code changes a value before asking for a fresh verdict, it asks after
+    /// that render, i.e., in OnAfterRenderAsync; asked at once, the field would judge the old value.
+    /// </remarks>
+    /// <param name="changedField">The field the form reported as changed, or null when the change happened outside the fields.</param>
+    /// <param name="dependentFields">The fields whose rules read other fields. A field which is not rendered yet is null.</param>
+    public static async Task RevalidateAsync(IFormComponent? changedField, params IFormComponent?[] dependentFields)
+    {
+        foreach (var field in dependentFields)
+        {
+            if (field is null || ReferenceEquals(field, changedField))
+                continue;
+
+            if (field.Touched || field.HasErrors)
+                await field.Validate();
+        }
+    }
+}