feat(api): add validation to prevent duplicate accounts from being added

src/FiscalOS.API/Accounts/Add/Request.cs

@@ -0,0 +1,23 @@
+
+namespace FiscalOS.API.Accounts.Add;
+
+public record Request : IValidatableObject
+{
+  public string PlaidInstitutionId { get; init; } = string.Empty;
+  public string PlaidAccountId { get; init; } = string.Empty;
+
+  public IEnumerable<ValidationResult> Validate(ValidationContext validationContext)
+  {
+    if (string.IsNullOrWhiteSpace(PlaidInstitutionId))
+    {
+      var fieldName = nameof(PlaidInstitutionId);
+      yield return new($"The {fieldName} field is required.", [fieldName]);
+    }
+
+    if (string.IsNullOrWhiteSpace(PlaidAccountId))
+    {
+      var fieldName = nameof(PlaidAccountId);
+      yield return new($"The {fieldName} field is required.", [fieldName]);
+    }
+  }
+}