[PM 19727] Update InvoiceUpcoming email content (#6168)

* changes to implement the email

* Refactoring and fix the unit testing

* refactor the code and remove used method

* Fix the failing test

* Update the email templates

* remove the extra space here

* Refactor the descriptions

* Fix the wrong subject header

* Add the in the hyperlink rather than just Help center

src/Core/Billing/Extensions/InvoiceExtensions.cs

@@ -0,0 +1,76 @@
+using System.Text.RegularExpressions;
+using Stripe;
+
+namespace Bit.Core.Billing.Extensions;
+
+public static class InvoiceExtensions
+{
+    /// <summary>
+    /// Formats invoice line items specifically for provider invoices, standardizing product descriptions
+    /// and ensuring consistent tax representation.
+    /// </summary>
+    /// <param name="invoice">The Stripe invoice containing line items</param>
+    /// <param name="subscription">The associated subscription (for future extensibility)</param>
+    /// <returns>A list of formatted invoice item descriptions</returns>
+    public static List<string> FormatForProvider(this Invoice invoice, Subscription subscription)
+    {
+        var items = new List<string>();
+
+        // Return empty list if no line items
+        if (invoice.Lines == null)
+        {
+            return items;
+        }
+
+        foreach (var line in invoice.Lines.Data ?? new List<InvoiceLineItem>())
+        {
+            // Skip null lines or lines without description
+            if (line?.Description == null)
+            {
+                continue;
+            }
+
+            var description = line.Description;
+
+            // Handle Provider Portal and Business Unit Portal service lines
+            if (description.Contains("Provider Portal") || description.Contains("Business Unit"))
+            {
+                var priceMatch = Regex.Match(description, @"\(at \$[\d,]+\.?\d* / month\)");
+                var priceInfo = priceMatch.Success ? priceMatch.Value : "";
+
+                var standardizedDescription = $"{line.Quantity} × Manage service provider {priceInfo}";
+                items.Add(standardizedDescription);
+            }
+            // Handle tax lines
+            else if (description.ToLower().Contains("tax"))
+            {
+                var priceMatch = Regex.Match(description, @"\(at \$[\d,]+\.?\d* / month\)");
+                var priceInfo = priceMatch.Success ? priceMatch.Value : "";
+
+                // If no price info found in description, calculate from amount
+                if (string.IsNullOrEmpty(priceInfo) && line.Quantity > 0)
+                {
+                    var pricePerItem = (line.Amount / 100m) / line.Quantity;
+                    priceInfo = $"(at ${pricePerItem:F2} / month)";
+                }
+
+                var taxDescription = $"{line.Quantity} × Tax {priceInfo}";
+                items.Add(taxDescription);
+            }
+            // Handle other line items as-is
+            else
+            {
+                items.Add(description);
+            }
+        }
+
+        // Add fallback tax from invoice-level tax if present and not already included
+        if (invoice.Tax.HasValue && invoice.Tax.Value > 0)
+        {
+            var taxAmount = invoice.Tax.Value / 100m;
+            items.Add($"1 × Tax (at ${taxAmount:F2} / month)");
+        }
+
+        return items;
+    }
+}